Add help search to JS Bin

Keyboard accessible (also via mouse from the help menu) - start typing, get some results.

![help-search mp4](https://cloud.githubusercontent.com/assets/13700/3937347/1e8e7234-24ac-11e4-92e0-990cd250b42b.gif)

Things that aren't amazing: 

- Keyboard nav is only for the search results, not the whole menu (but then it would get trapped in the input, in a way, this is good)
- Our seach is *very* fuzzy - something that can't easily be changed, but worth just noting
- If the results are taller than the page, there's no handing for scrolling

+1 / -1? Any tweak requests will be minimal.