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.
* Add next/config
* Set config on server start / client render
* Add documentation for next/config
* Add next/config support for next export
* Fix test
* Use the correct name
* Set default to empty object on the client side
* Add config tests
* Rename config to runtimeConfig