Add extensions option to enable configurable file extension matching

[i-like-robots]

Sometimes it is useful to resolve files from bower.main: [] which are not JS files. This PR adds an extensions option which enables the configuration of the file extension/s to match.

Example use case

The latest versions of the sass-loader package (7.x) builds upon more of Webpack's module resolution algorithm rather than following the simple algorithm built into node-sass.

https://github.com/webpack-contrib/sass-loader/blob/master/CHANGELOG.md#700-2018-04-13

This means to use sass-loader effectively you usually need to add some extra resolve configuration to the Sass rule in order to resolve .scss and .sass files:

rules: [
  {
    test: [/\.s(c|a)ss$/],
    resolve: {
      extensions: ['.scss', '.sass']
    },
    use: [ 'sass-loader' ]
  }
]

However when using the new sass-loader package with this plugin to help resolve packages installed using Bower currently only .js files will be matched so setting the file extension as shown above has no effect.

My changes are intended to make it possible to use this plugin in situations like this where I want to resolve non-JavaScript files, e.g.:

rules: [
  {
    test: [/\.s(c|a)ss$/],
    resolve: {
      plugins: [ new BowerResolvePlugin({ extensions: ['.scss', '.sass'] }) ],
      extensions: ['.scss', '.sass']
    },
    use: [ 'sass-loader' ]
  }
]

---

I hope this makes sense! This is only a suggestion, if you are happy with it then I will update the readme as well.

cc webpack-contrib/sass-loader#556

[JakeChampion]

Hi @linaspasv, is there any chance this could get merged in at all? :-)