Added support for multiple (stacked) reveals. Added options 'allowMulti' [default: false] and 'offsetMulti' [default: 10] to support this new behavior.

A limitation I see with this patch is that all subsequent modals must use allowMulti as well, or the whole stack will close. I am not certain what the best approach is when several Reveals with allowMulti set to true and false co-exist in a single stack.

Something also seems to mess up with the offset during responsive redrawing. If this is a concern, the current best solution is to simply not use the offset. The visual impact is most important on a desktop site, I don't think it is important or useful on mobile.