Allow callers to define psuedoclasses via callback.

Move :root handling into interpretor.

Correct pseudoclass to pseudoelement rewrites to apply to only rightmost selector.

I previously got the semantics of my datastructures backwards: the head is the rightmost selector.

Lower psuedoclasses & namespaces throughout CSS selectors.

Previously I just applied to them to the rightmost.

Rewrite parsed namespaces according to @namespace rules.

Correct legacy :before/:after syntax.

Support lowering single-arg psuedoclasses.

Implement psuedoclass rewrite rules.