add tree view vertical/horizontal split bindings

[sheldonkwok]

#685

[bronson]

I'm a little worried about the precedent this sets... Should tree-view support ctrl-w ctrl-l? How about / to search?

That said, I think this is great. There doesn't seem to be any real downside. 👍

[bronson]

After further thought, I've changed my mind... This is the perfect use for ~/.atom/keymap.cson.

I do think it does make sense to collect these ideas into a docs file and include them with vim-mode. For now, I added an Interesting Ideas section to v-m-n's readme.

[sheldonkwok]

Sounds good, thanks for the input