Display test coverage output

This code will run `go test` and colorize your buffer to indicate test coverage results.

## TODO
- [ ] Fix up the parser, it's gross
- [ ] Indicate running, in case it's a large test suite
- [x] Ensure temp files are cleaned up
- [x] Handle and display any errors
- [ ] Display textual coverage results
- [ ] Make highlighting to the end of line consistent
- [ ] Write some tests
- [x] Keystroke toggle

![coverage2](https://cloud.githubusercontent.com/assets/143/2817003/1a45434e-ced3-11e3-9ba5-0d2f0bc13e47.gif)