VIM-265 Add window split commands [Rebased]

#28 Rebased on top of current master; @ghuh I picked this up because it's a feature I really want, I hope you don't mind.

@carymrobbins the ```:q[uit]``` behavior is an artifact of the IntelliJ OpenAPI; there is no way to close only a single instance of a file, you must close all of them.