Whitespace removed from CSS file when `@source`, `@config`, or `@plugin` is used

[thecrypticace]

Using @source in CSS file causes Prettier to remove whitespace between all rules. This happens because of our re-parsing. We're using the parsed AST object instead of a string which causes prettier to reformat those nodes.

Unfortunately this also somehow triggers Prettier to remove all blank whitespace in the file — need to figure out why which will need some source diving in Prettier itself.

Affects the main branch but not 0.6.x because that change isn't released yet.

Caused by #387