[selectors] :sorted pseudoclass for HTML

Hixie recently added the ability to declaratively sort tables to HTML. To
support these on the CSS side, it would be convenient to define a :sorted
pseudoclass, along with :sorted(ascending) and :sorted(descending), which
apply to table cells in sorted columns. The relevant bug is at <
https://www.w3.org/Bugs/Public/show_bug.cgi?id=20522>. We can just defer to
the host language to define what elements are sorted at any given time.

Anyone have any objections to us adding these to Selectors 4?

~TJ