UX overhaul to feed builder.

Builds upon changes in #27, will merge them together in this PR and close that one. Too much code overlap to do them separately, because @ngokevin is smart and DRY.

r? @ngokevin @spasovski

## Before
![screen shot 2014-07-24 at 2 40 02 pm](https://cloud.githubusercontent.com/assets/23885/3693691/8ccd7ac2-136a-11e4-8b16-d71e73a81b73.png)

## After
![screen shot 2014-07-24 at 2 39 23 pm](https://cloud.githubusercontent.com/assets/23885/3693694/908bf5c6-136a-11e4-82e9-e7c4f8090474.png)



input = keypress + change + <paste>, basically. Plus some other contenteditable stuff.