smoother realtime update of rendered markdown display

I was using morkdown to edit the mmckegg/become readme, but every time I typed, the rendered markdown would jump around as images reloaded. I suddenly realised that [become](http://github.com/mmckegg/become) could be exactly what this project needs.

"Become" works like `element.innerHTML` except that it only updates elements that have actually changed. That way the images won't have to continuously reload and the scroll position will remain constant.