Allow transform options
Allow transform options

Such as the following code can specify some transform options:

``` ruby
options = {modules: 'amd'} # Specify options which you like

Sprockets.register_transformer 'text/ecmascript-6', 'application/javascript', Sprockets::ES6.new(options)
```

This approach comes from [sprockets 4.x branch](https://github.com/sstephenson/sprockets/blob/6e08a268/lib/sprockets/es6to5_processor.rb).

blasterpal/sprockets-es6,rstacruz/sprockets-es6,Valve/sprockets-es6,louisjimenez/sprockets-es6,mulvaney/sprockets-es6,TannerRogalsky/sprockets-es6,zackp30/sprockets-es6