allow options.content to be a function returning html

If `options.content` is a function, call it to get the drop's html, and then do so on each subsquent open.  Bootstrap tooltips allow this and i used this ability heavily.  I find myself destroying and recreating Drops/Tooltips if the message needs to change, and this is easier and more efficient.  I'm glad to update the Docs if you think this enhancement makes sense.

@adamschwartz