Allow date boundaries

(Note I plan to submit this code within the next few days/weeks)

I think it would be nice if I could pass a `startDate` and/or an `endDate` to the settings and have the month picker auto grey out or remove the months that are not selectable. 

Currently to do this I have to add event listeners myself and change the disabledMonths option.  It's a bit tedious to have to keep updating that.  

Deos this seem like a good change to you as well?