jump motions (ctrl-o / ctrl-i)

I've been playing around with implementing [jump motions](http://vimhelp.appspot.com/motion.txt.html#jump-motions), to jump back and forth between previous cursor positions.

It is not finished. Before going any further I would prefer to get some feedback whether this makes sense.

Basically, `atom.workspace.vimState.jumpList` will keep track of the history of cursor positions across buffers in the current workspace. Entries on this list are added when a jump-motion is executed (gg, G, /, and some other commands).
Then ctrl-o, ctrl-i is used to jump to older and newer positions, that could be in the same buffer, or in a different buffer - using atom.open in this case, to switch to the appropriate tab, or open the file if it was closed.

There are a few things pending, like creating an entry in the jumplist when switching buffers/tabs, which I'm not sure how to implement at the moment.

Any feedback on this? ideas?