Click to view visual history of an example

[lencioni]

@lelandrichardson and I were chatting with Alex from Facebook and he suggested adding a feature to Happo that would allow you to click on a screenshot (or maybe draw a rectangle on one) and it will bring you to an interface that would show you the visual history of that example with links to tak you to the commit or PR that introduced each of the changes along the way.

I think to pull this off we will first need to do #178 in addition to keeping track of some metadata somehow to tie them together. It would also be ideal to only show you visual history for master, which would require you to run Happo on changes to master.