support for pressing enter in ace-jump-buffer menu to toggle to last buffer

Howdy,

Thanks for making this. I really like this package for selecting from a giant list of buffers. So I did a `(define-key global-map (kbd "C-x b") 'ace-jump-buffer)`. Although I have a lot of muscle memory built up for quickly pressing enter to switch (toggle) to the last buffer I was on (from the default `C-x b` behavior). So I made the attached tweaks. I don't know if it's out side the bounds of the features you want to support, but I thought I'd bring it up just in case. 

**This probably isn't ready to go as-is** I am not sure if there's a better way to do this (although I know there's probably a better name for the `defcustom`). Thanks again.