* Let webpack2 to handle ES2015 module system
Since Node.js can't do that, we need to transpile
ES2015 module system in the emit-file-loader.

* Use sourceMaps only in dev.

* Introduce a transform option to emit-file-loader
So, we can move our ES2015 transpile code with that option.

* Remove unwanted argument options.

* Update comments.

* Use dev flag instead of NODE_ENV