"of" clause of :nth-(last-)child is not fully parsed

See http://drafts.csswg.org/selectors-4/#the-nth-child-pseudo
Currently, in e.g. `:nth-child(n+3 of li.important)`, the `li.important` portion is just returned as part of a `content` string and doesn't get parsed into a selector subtree.
