Selectors should support XML Namespace URIs, too

[caitp]

Well, maybe they should.

I left a comment on #1 (comment):

I don't have time right now to give a proper review, but I was wondering if it might also make sense to add selectors for XML namespaces, to assist in supporting SVG and MathML stuff? I guess if we need that it could be added later on, but it seems like a good idea

It may or may not be worth it, but it seems like a potentially useful thing