Support @supports

[JakeQZ]

This is the de-facto standard now for determining 'browser' or email client CSS support for a specific feature.

We should probably process these CSS rules as we do @media rules so that their placement in the <style> element containing the uninlinable CSS is correct (and those that can't possibly apply can be eliminated).

If anyone can think of anything more clever we can do with @supports rules, please send your suggestions on a postcard.