Close #152 - Add support for reading shebang

![shebang](https://cloud.githubusercontent.com/assets/1694055/3393915/946ef346-fcd1-11e3-95ba-c93740c61fa0.gif)
- [x] Add `CodeContext#shebangCommand()`
- [x] Fall back to grammar.coffee's command definition
- [x] Ensure to always use command over shebang/grammar (when running with options)
- [x] Add specs for the new `CodeContext#shebangCommand()`

Add a test that involves selected text.