Better `style` sanitizing

[fluffy-critter]

Certain web CMSes (*cough* Publ *cough*) make use of style attributes in ways that tend to break some things when they're completely filtered out. But we don't want every possible style to be whitelisted. It's a good idea to parse out the CSS rules (which is a pain) and only allow in certain whitelisted ones, like background-image, border-shape, and float/clear.