add vi-like-bindings

[Jengah]

https://github.com/Mckinsey666/bullet/pull/21 was prematurely closed. #23 implemented a vehicle for allowing multiple keys to be registered to the same method, but did not actually add the vi-like bindings that were requested in Issue https://github.com/Mckinsey666/bullet/issues/13

This branch actually implements the feature requested. Thanks @rcfox for #23 making it way simpler.

[Jengah]

I've rebased off of your latest changes. Any reason this can't go in @Mckinsey666?

This will add the functionality that @dkasak was asking for.