Completion handler

[andreyrd]

Added a completion handler to the dropdown, as discussed in #55

I kept action as the last parameter to drop(), so that we don't break anyone using trailing closure syntax. Based on a compile of my own project that uses SwiftyDrop, this doesn't seem to be a breaking change, but I'm not sure how adding this optional parameter will affect other projects.

[morizotter]

Thanks! I think it's a good approach. But I have a same concern about existing action. It'll maybe good for adding deprecated for exiting action .. mm.

[andreyrd]

Ah, right now action handles tap, while completion handles swipe or timeout. Maybe for a v5, completion could handle tap, swipe, and timeout!