Chore: Rid CSS output of empty :root blocks and comments

[erikjung]

This addresses #80 by:

• Adding a new PostCSS plugin that strips comments out of the transpiled CSS
• Adding a new PostCSS plugin that strips empty :root {} blocks out of the transpiled CSS

[tylersticka]

Boy, I can see now why we usually go with the /* 1 */ format for comments. The key/value list is harder to read (and more repetitive, since you now have to keep the variable name up-to-date in two places).

I would prefer if we used postcss-discard-empty. Paired with the plugin you already added in this PR, it should allow us to continue writing comments in a readable, maintainable way without fear of littering the final stylesheet with empty selectors.

[erikjung]

@tylersticka I didn't know about postcss-discard-empty, and this proposed solution sounds better to me as well.

[tylersticka]

LGTM! 👍