[RFC] Keyboard selection

Here is a simple implementation enabling keyboard navigation of results as is done by, e.g., [`rustdoc`](http://static.rust-lang.org/doc/master/std/index.html?search=cell).

I still don't consider this finished work as I worry that it might interfere too strongly with scrolling (which is more likely to be needed in hoogle results than with rustdoc) and the styling could be improved.

Thoughts?