Support comma separated architectures in make

[liamwhan]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Please describe your issue:
This is a fairly minor issue, as it can be avoided by building architectures separately or building with --arch=all but its preventing electron-forge from accepting arguments that electron-packager supports.

When invoking make with --arch=x64,ia32 electron-forge packages OK and then searches for a packaged folder in the out dir named <project_name>-x64,i32

System Info:

OS: Ubuntu 16.04 x64
Node Version: 7.4.0
npm version: 4.5.0

Console output when you run electron-forge with the environment variable DEBUG=electron-forge:*. (Instructions on how to do so here). Please include the stack trace if one exists.

WARNING: DEBUG environment variable detected.  Progress indicators will be sent over electron-forge:lifecycle
  electron-forge:lifecycle Process Started: Checking your system +0ms
  electron-forge:lifecycle Process Succeeded: Checking your system +12ms
  electron-forge:runtime-config setting key: verbose to value: false +5ms
WARNING: DEBUG environment variable detected.  Progress indicators will be sent over electron-forge:lifecycle
  electron-forge:lifecycle Process Started: Resolving Forge Config +0ms
  electron-forge:project-resolver searching for project in: /home/liam/Documents/Valence +1ms
  electron-forge:project-resolver electron-forge compatible package.json found in /home/liam/Documents/Valence/package.json +6ms
  electron-forge:lifecycle Process Succeeded: Resolving Forge Config +6ms
  electron-forge:require-search searching [ '../makers/linux/deb.js',
  '../makers/generic/deb.js',
  'electron-forge-maker-deb',
  'deb',
  '/home/liam/Documents/Valence/deb',
  '/home/liam/Documents/Valence/node_modules/deb',
  '/home/liam/.config/yarn/global/node_modules/electron-forge/dist/makers/linux/deb.js',
  '/home/liam/.config/yarn/global/node_modules/electron-forge/dist/makers/generic/deb.js',
  '/home/liam/.config/yarn/global/node_modules/electron-forge/dist/api/electron-forge-maker-deb',
  '/home/liam/.config/yarn/global/node_modules/electron-forge/dist/api/deb',
  '/home/liam/Documents/Valence/deb',
  '/home/liam/Documents/Valence/node_modules/deb',
  '/home/liam/.config/yarn/global/node_modules/electron-forge/dist/api/makers/linux/deb.js',
  '/home/liam/.config/yarn/global/node_modules/electron-forge/dist/api/makers/generic/deb.js',
  '/home/liam/.config/yarn/global/node_modules/electron-forge/dist/api/node_modules/electron-forge-maker-deb',
  '/home/liam/.config/yarn/global/node_modules/electron-forge/dist/api/node_modules/deb',
  '/home/liam/Documents/Valence/deb',
  '/home/liam/Documents/Valence/node_modules/deb' ] relative to /home/liam/.config/yarn/global/node_modules/electron-forge/dist/api +1ms
  electron-forge:require-search testing ../makers/linux/deb.js +2ms
We need to package your application before we can make it
  electron-forge:lifecycle Process Started: Preparing to Package Application for arch: x64,ia32 +20ms
  electron-forge:project-resolver searching for project in: /home/liam/Documents/Valence +0ms
  electron-forge:project-resolver electron-forge compatible package.json found in /home/liam/Documents/Valence/package.json +1ms
  electron-forge:hook could not find hook: generateAssets +8ms
  electron-forge:hook could not find hook: prePackage +0ms
  electron-forge:packager packaging with options { asar: false,
  overwrite: true,
  ignore: 
   [ '.idea',
     'src/vendor/materialize/fonts',
     'src/vendor/materialize/sass',
     'dist',
     'vid',
     'test',
     'install.sh',
     'Gulpfile.js',
     'license-concat.js',
     '.nvmrc',
     '.gitignore',
     '.nyc_output',
     'logo/ValenceLogo.ai' ],
  icon: 'build/icon.ico',
  electronVersion: '1.6.6',
  afterCopy: [ [Function], [Function], [Function], [Function] ],
  afterExtract: [],
  dir: '/home/liam/Documents/Valence',
  arch: 'x64,ia32',
  platform: 'linux',
  out: '/home/liam/Documents/Valence/out',
  quiet: true } +0ms
  electron-forge:lifecycle Process Succeeded: Preparing to Package Application for arch: x64,ia32 +5s
  electron-forge:lifecycle Process Started: Compiling Application +1ms
  electron-forge:lifecycle Process Succeeded: Compiling Application +3s
  electron-forge:lifecycle Process Started: Preparing native dependencies +1ms
  electron-forge:lifecycle Process Succeeded: Preparing native dependencies +59ms
  electron-forge:lifecycle Process Started: Packaging Application +1ms
  electron-forge:lifecycle Process Succeeded: Packaging Application +11s
  electron-forge:lifecycle Process Started: Preparing to Package Application for arch: x64 +1ms
  electron-forge:lifecycle Process Succeeded: Preparing to Package Application for arch: x64 +1s
  electron-forge:lifecycle Process Started: Compiling Application +1ms
  electron-forge:lifecycle Process Succeeded: Compiling Application +2s
  electron-forge:lifecycle Process Started: Preparing native dependencies +1ms
  electron-forge:lifecycle Process Succeeded: Preparing native dependencies +53ms
  electron-forge:lifecycle Process Started: Packaging Application +0ms
  electron-forge:hook could not find hook: postPackage +8s
  electron-forge:lifecycle Process Succeeded: Packaging Application +0ms
Making for the following targets:
  electron-forge:hook could not find hook: preMake +1ms

An unhandled rejection has occurred inside Forge:
Couldn't find packaged app at: /home/liam/Documents/Valence/out/Valence-linux-x64,ia32
Error: Couldn't find packaged app at: /home/liam/Documents/Valence/out/Valence-linux-x64,ia32
    at _loop$ (/home/liam/.config/yarn/global/node_modules/electron-forge/dist/api/make.js:321:29)
    at tryCatch (/home/liam/.config/yarn/global/node_modules/regenerator-runtime/runtime.js:65:40)
    at Generator.invoke [as _invoke] (/home/liam/.config/yarn/global/node_modules/regenerator-runtime/runtime.js:303:22)
    at Generator.prototype.(anonymous function) [as next] (/home/liam/.config/yarn/global/node_modules/regenerator-runtime/runtime.js:117:21)
    at tryCatch (/home/liam/.config/yarn/global/node_modules/regenerator-runtime/runtime.js:65:40)
    at maybeInvokeDelegate (/home/liam/.config/yarn/global/node_modules/regenerator-runtime/runtime.js:365:18)
    at Generator.invoke [as _invoke] (/home/liam/.config/yarn/global/node_modules/regenerator-runtime/runtime.js:277:32)
    at Generator.prototype.(anonymous function) [as next] (/home/liam/.config/yarn/global/node_modules/regenerator-runtime/runtime.js:117:21)
    at Generator.tryCatcher (/home/liam/.config/yarn/global/node_modules/bluebird/js/release/util.js:16:23)
    at PromiseSpawn._promiseFulfilled (/home/liam/.config/yarn/global/node_modules/bluebird/js/release/generators.js:97:49)
    at Promise._settlePromise (/home/liam/.config/yarn/global/node_modules/bluebird/js/release/promise.js:574:26)
    at Promise._settlePromise0 (/home/liam/.config/yarn/global/node_modules/bluebird/js/release/promise.js:614:10)
    at Promise._settlePromises (/home/liam/.config/yarn/global/node_modules/bluebird/js/release/promise.js:693:18)
    at Async._drainQueue (/home/liam/.config/yarn/global/node_modules/bluebird/js/release/async.js:133:16)
    at Async._drainQueues (/home/liam/.config/yarn/global/node_modules/bluebird/js/release/async.js:143:10)
    at Immediate.Async.drainQueues (/home/liam/.config/yarn/global/node_modules/bluebird/js/release/async.js:17:14)

What command line arguments are you passing?

electron-forge make --platform=linux --arch=x64,ia32

What does your config.forge data in package.json look like?

"config": {
    "forge": {
      "github_repository": {
        "owner": "ValenceEncoder",
        "name": "Valence",
        "draft": true,
        "prerelease": true
      },
      "publish_targets": {
        "win32": [
          "github"
        ]
      },
      "make_targets": {
        "win32": [
          "squirrel"
        ],
        "darwin": [
          "zip"
        ],
        "linux": [
          "deb"
        ]
      },
      "electronPackagerConfig": {
        "ignore": [
          ".idea",
          "src/vendor/materialize/fonts",
          "src/vendor/materialize/sass",
          "dist",
          "vid",
          "test",
          "install.sh",
          "Gulpfile.js",
          "license-concat.js",
          ".nvmrc",
          ".gitignore",
          ".nyc_output",
          "logo/ValenceLogo.ai"
        ],
        "icon": "build/icon.ico",
        "electronVersion": "1.6.6",
        "overwrite": true
      },
      "electronWinstallerConfig": {
        "name": "Valence",
        "setupExe": "valence-encoder-setup.exe",
        "description": "Valence is a graphical interface video encoder that uses Electron and FFMPEG."
      },
      "electronInstallerDebian": {},
      "electronInstallerRedhat": {},
      "windowsStoreConfig": {
        "packageName": "",
        "name": "valence"
      }
    }
  }

Please provide either a failing minimal testcase (with a link to the code) or detailed steps to
reproduce your problem. Using electron-forge init is a good starting point, if that is not the
source of your problem.