Find and replace

Adds code search with find and replace.

![screen shot 2014-03-06 at 6 22 46 pm](https://f.cloud.github.com/assets/1577766/2353367/73e40d70-a59f-11e3-86fe-24d501aaff1b.png)

![screen shot 2014-03-06 at 6 23 09 pm](https://f.cloud.github.com/assets/1577766/2353369/7810d1bc-a59f-11e3-8f6c-cc4c102951a3.png)

- <code>Cmd-F</code> Find
- <code>Cmd-G</code> Find next
- <code>Shift-Cmd-G</code> Find previous
- <code>Cmd-Option-F</code> Find and replace
- <code>Esc</code> Clear match highlights

Closes https://github.com/mapbox/tm2/issues/118