* 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
* Remove default next pages compilation from the main babel-loader.
This will fix the issue when the user ignore node_modules
via our .babelrc option.
* Change babel-loaders 'query' to 'options'.
That's what's supported/recommended in webpack 2
* compile _document using webpack
* don't emit the bundle file of _document.js
* exclude _document.js from minChunks of CommonsChunkPlugin
* handle creation/removal of pages/_document.js
* improve path handlings
* Create a babel-preset for plugins and presets we use.
Anyone could get it via 'next/babel'
* Remove 'use strict' from the source.
* Add next/router to alias resolver in our babel preset.
