Media query support

* Use the new `media_types` information from `css_parser` to handle `@media`
** Handle unmergable rules (like `@media` and `:hover`) by leaving them in the `<style>` tag. Assume special cases like this are nice-to-haves and not critical functionality.
* Fix failing tests by re-instituting CSS shorthands. Mail loading speed matters so this seems like a sane default.

**Note:** Requires a `css_parser` with https://github.com/alexdunae/css_parser/pull/42 applied. before tests will pass.