Added support to resize to 2/3

I've added support to allow for resizing windows to fit 2/3 of the screen to complement the 1/3 fit. 
See https://github.com/eczarny/spectacle/issues/75

~~I wasn't able to figure out how to set the default key codes. I was using control + shift + alt + arrow key to move to left or right 2/3rds of the screen.~~