* 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.