unite.vim source

This doesn't adversely affect users who don't have unite, because it will never be loaded for them. For those that do have unite, they get:

    :Unite tmuxcomplete

And then you can get strings from tmux even if something like YCM clobbers your `<c-x><c-u>`. 

If you plan to merge this, I will add brief documentation and clean up the commit message, etc.