Handle !important CSS declarations (fixes #49)

This was a little trickier than I thought because "!important" allows individual properties within a single rule to have a different priority than the selector would indicate. But my solution is pretty straightforward: split the rule and pull out the "important" properties.

Looping over that tuple is clever and I'll give that a try, but I worry this function is getting hard to read. This line is already inside like 3 nested `for` loops.

Good call on rule_index.