Add new command to open NERDTree in the root of a VCS repository.

[PhilRunninger]

Implements #335

I'm not sure this is the best way to implement this request. It uses a new command - :NERDTreeVCS. Another way would be to add an option similar to Ctrl-P's g:ctrlp_working_path_mode.

@lifecrisis , Do you have a preference?

[lifecrisis]

Something about this doesn't feel right. I almost want to argue that this should be a plugin.

I'd like to leave this and come back to it later.