Depends on https://github.com/zeit/next-plugins/pull/228
Failing tests are expected as `@zeit/next-css` has to be updated/released first.
This implements rendering of `.css` chunks. Effectively removing the custom document requirement when adding next-css/sass/less/stylus.
* Compile pages to .next/static/<buildid>/pages/<page>
* Fix test
* Export class instead of using exports
* Use constant for static directory
* Add comment about what the middleware does
The prepares for next-server.
I also took this as an opportunity to get all build directory paths from a single location, as they were previously scattered across webpack/babel plugins and loaders.
