Vim bindings: add Option-[ and Option-] to do prevd/nextd

I was jealous reading in the fish mailing list last week that there was a key code that can do nextd/prevd functionality in the default key bindings. Because I use the vi bindings, I am missing out!

I brought over the 'nextd/prevd' functionality in Command Mode under Option-[ and Option-], which come across as the unicode characters “ and ‘ in OSX. I've used the unicode values of these as the bindings, but I doubt those will port over onto non-OSX? I'm not sure what to do about that, but it works well in OSX! :) I'm certainly open to alternative keys, but these seemed to be the most natural to me.

Note that the default bindings actually implement "nextd-or-forward-word" but I'm suggesting "nextd force-repaint" here. It seems to fit better with the style of vim's Command Mode.