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 
Seems like you'd want this for the "Cancel" button also. You could have `self.addEventListener('close', ...)` and have both buttons emit that however.