Checked dates feauture - option to mark specific date in calendar view as checked for additional styling.
May be useful to show holidays, birthdays or any other events in a datepicker view.

Already finished:
* added 'checkedDates' option and 'data-checked-dates' attribute support. Dates should be specified in the same format as set to datepicker.
* 'checked' class is added to every day included in a checkedDates.
* added data-date attribute for month element to simplify month date checking.
Added 'checked' class for months included in checkedDates.
Added default styling for 'checked' dates.