Adds preview plugin for gallery

Fixes #286 

Adds preview plugin for gallery building off of #249 

![screen shot 2014-06-16 at 5 17 30 pm](https://cloud.githubusercontent.com/assets/1656824/3294994/d709d640-f5b4-11e3-9a2b-b78a2c6ae008.png)

While I know this wasn't part of this PR, this method makes me nervous for a couple of reasons (untested / funky method signature), and seems like it's primed for a bit of a refactor.

I don't want to hold up this PR for that, but if you felt inclined to tackle this... :+1: