add mkt-prompt element

![screen shot 2014-12-18 at 6 05 00 pm](https://cloud.githubusercontent.com/assets/674727/5499226/7038bf22-86e0-11e4-9834-51fe33050cce.png)

@mstriemer @spasovski 
well, y'all already have a polyfill for `document.registerElement`. but yeah, that's not an excuse to add that.

callbacks are fine. they're fine.