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 
I'd rather see an event get triggered that have DOM elements become promises. I like the idea and maybe it makes sense to have a `promise` or `submitted` property but perhaps you don't want the modal to close on submission and it can fire more than once?

If there was some other event that you used, I guess it could be `submit` still, then you could probably have a default handler on the element that handles dismissing the modal (which can be skipped if `e.defaultPrevented`). Then whatever is wrapping this can call `e.preventDefault()` if it doesn't want the modal to close.