Added missing support for AppX fileAssociations

[danielweck]

I tested this change using the following package.json ElectronBuilder build config:

    "fileAssociations": [
      {
        "ext": "ext1",
        "role": "Viewer",
        "mimeType": "application/ext1+zip"
      },
      {
        "ext": "ext2",
        "role": "Viewer",
        "mimeType": "application/ext2+zip"
      }
    ],

I am running a Windows 10 Pro Virtual Machine (VirtualBox), in "developer mode" so I can install my test AppX which is code-signed (via electron-builder's own certificateFile config) using a self-signed PFX certificate (generated using electron-builder create-self-signed-cert -p NAME) which I added in my "Trusted Root Certification Authorities" (using mmc.exe and a "Certificates" snap-in).

Once my test AppX is installed (simply by double-clicking on it from the Windows file explorer), I am able to double-click or "open with" from the file explorer the newly-registered ext1 and ext2 file extensions.

Once I uninstall the AppX, the file extensions are immediately unregistered, as expected.

[danielweck]

Update: I added test/integration details in the PR description.

[develar]

Thanks a lot.