Key bindings support

Default keys are set to following: (don't seem to conflict with anything)

 - `ctrl-shift-[`: Convert Spaces To Tabs
 - `ctrl-shift-]`: Convert Tabs To Spaces

![screenshot from 2014-07-15 21 23 02](https://cloud.githubusercontent.com/assets/7157049/3590880/d9d654ec-0c5d-11e4-9ec8-e301e08e6850.png)