Add support for more selectors

Hello! As part of my work on [Happybara](https://github.com/cstrahan/happybara), I've implemented a CSS selector parser and a converter from CSS to XPath:

https://github.com/cstrahan/happybara/blob/to-query/happybara/src/Happybara/XPath.hs#L40
https://github.com/cstrahan/happybara/blob/to-query/happybara/src/Happybara/CSS.hs

If you're interested, I'd like to fold my implementation into HandsomeSoup.

If you wouldn't mind taking a dependency on [`hxt-xpath`](http://hackage.haskell.org/package/hxt-xpath), I think we should be able to use my css-to-xpath conversion directly, using [`getXPathTreesInDoc`](http://hackage.haskell.org/package/hxt-xpath-9.1.2.1/docs/Text-XML-HXT-XPath-Arrows.html#v:getXPathTreesInDoc).

Thoughts?