Added shortcuts for opening top, left, and bottom panes, and proper OS labels for the menu.

I looked around and couldn't see available shortcuts for opening the different terminals in different panes (top, left, bottom) so I created it. I also added proper OS labels for the shorcuts on the menu (ie, so that if you're on a mac you see the actual icons for the shortcuts next to the label). (I really liked this plugin, so this is my way of saying thanks!)