Implement CSS modules

[devongovett]

#6

[height]

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[nicksrandall]

I was just poking around in this repo to see if I could add help by adding basic support for CSS modules and then ~20 minutes later you submit this PR. haha. Anyway, this is exciting!

Are you looking for any additional help?

[devongovett]

@nicksrandall thanks for the offer! I should be able to finish this PR up pretty quick, but there is plenty of stuff to do in this project in general if you still want to help. I created some issues already, e.g. properties we don't parse yet, or new CSS features we don't downlevel yet. Happy to help if you have questions!