key bindings by OS

inspired from:

Rename keycaps file to only work on os x

https://github.com/defunkt/zen/commit/81d6b5b1061567edb7ba646856a57e61acff924f

Add linux and windows keybindings

https://github.com/defunkt/zen/commit/5f67834a1fb5974c1e7bbda0778448d5b4bd56db