[selectors4] Comma-separated attribute value selectors

There's an issue in the Selectors spec [1] on adding comma-separated values
to the attribute selectors, as syntactic sugar, e.g.

   [rel=next,prev] { ... }

[1] http://www.w3.org/TR/2013/WD-selectors4-20130502/#attribute-selectors

This seems pretty straightforward and desirable to me. Does anyone think
we *shouldn't* add this? 'Cuz otherwise I think we should add it. :)

~fantasai