Modernize project (#41)

* chore: Make index.js a es6 module, add umd/cjs build scripts
* feat: Add `plurals.js` es6 module with named exports
* style: Convert main module to es6/es2015 syntax
* Pin dependencies
* Add dist files to repo
* Prettify code and cleanup
* Kick buble, if you want to support older browsers use buble in your build
* Adapt urls
* Test on modern node version
* Force parens for arrow functions
* Build only commonjs

.gitignore

@@ -1,6 +1,4 @@
 node_modules
 npm-debug.log
-yarn-error.log
-.idea
-.vscode
 .DS_Store
+common.js

.prettierrc

@@ -0,0 +1,9 @@
+{
+  "useTabs": false,
+  "printWidth": 80,
+  "tabWidth": 2,
+  "singleQuote": true,
+  "arrowParens": "always",
+  "trailingComma": "es5",
+  "semi": false
+}

.travis.yml

@@ -1,5 +1,4 @@
 language: node_js
 sudo: false
 node_js:
-  - 5.2
-  - 4.1
+  - 8

README.md

@@ -20,7 +20,8 @@ Usage:
 ------
 
 ```JavaScript
-var translate = require('translate.js')
+import translate from 'translate.js'
+// var translate = require('translate.js')
 
 var messages = {
     translationKey: 'Translation value'
@@ -88,7 +89,8 @@ var messages = {
 
 Then bind the translation function to something short:
 ```JavaScript
-var translate = require('translate.js')
+import translate from 'translate.js'
+// var translate = require('translate.js')
 var t = translate(messages)
 ```
 
@@ -174,12 +176,13 @@ t('sheep', 21) => '21 kind'  // pluralize_IS(21) => 's'
 t('sheep', 13) => 'Baaahd luck'  // direct subkey hit 
 ```
 
-Translate.js comes with a predefined `pluralize` functions for [several languages](pluralize/). These can be required into your code as needed, like so:
+Translate.js comes with a predefined `pluralize` functions for [several languages](plurals.js). These can be imported into your code as needed, like so:
 
 ```js
-var pluralize_IS = require('translate.js/pluralize/is')
+import { plural_IS } from 'translate.js/plurals'
+// var plural_IS = require('translate.js/pluralize/is')
 var t = translate( messages_IS, {
-    pluralize: pluralize_IS
+    pluralize: plural_IS
 })
 ```