Revert "First pass at generating units within CSS media queries"

This reverts commit b960b41ec1f94c2360d9e0802d70d9c151d1bec4.
Implement media query support within the existing Rework plugin, take 2

This is a major refactor to clean up the code and add support for media
queries. This package now exports one funciton that is the Rework plugin
and does everything.

Media query configuration is provided by `options.mediaQueries`.
Move plugin back under `pureGrids.units()`
Update README for media-query refactor