Add several CSS 3 selectors and pseudo-classes

* Add selectors `^=` and `*=`.
* Add pseudo-classes `:default`, `:indeterminate`, `:invalid`,
  `:lang()`, `:in-range`, `:nth-last-of-type()`, `:only-child`,
  `:only-of-type`, `:optional`, `:out-of-range`, `:required`,
  `:root`, and `:valid`.
Merge pull request #75 from ianbollinger/add-selectors

Add several CSS 3 selectors and pseudo-classes