[selectors] :sorted pseudoclass for sorted tables

Now that HTML defines a way to sort tables, Hixie would like a
pseudoclass letting authors target based on this:
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=20522>

His proposal is for a :sorted pseudo-class, which also comes in a
functional version, :sorted( [ ascending | descending ] ).  He's
looking at defining it to match <th> elements that head a sorted
column.

Thoughts?  Any objections to me adding this to Selectors 4?

~TJ