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 
ah, caused by [mixins](https://github.com/mozilla/fireplace/blob/master/src/media/css/buttons.styl#L81). makes sense.