[feature request] Day.js support

Day.js is a lightweight and modular replacement for Moment.js. Its API is nearly the same as Moment's API.

It would be great if Pikaday supported it.

I don't know how Pikaday API would look like. Probably the .getMoment() the method would have to be replaced with something more generic.

It would be also good if Moment/Day.js reference could be passed explicitly to Pikaday. IMO it shouldn't internally depend on Moment/Day.js.