Added keyboard shortcuts à la WinMerge.

Hi Jamie, 
I've dreamt about this project. And when I woke-up, I had quite a few clean-up in mind. Then, I couldn't resist to create my first pull request. I hope you will be happy with the changes.
Kind regards,
Claude 

Link to updated demo : https://tkljh.s3.amazonaws.com/misc/mergely/html/ajax.html