Support marking day indexes in markedDates prop.

[dasanibroto]

Achieves something similar to what was being attempted by PR #965

Lets users put a day index in for marked date, and uses that for marking when appropriate. Priorities specific date marks over index mark, so users can, say, mark Mondays as disabled but then allow one specific Monday.

Example usage:

markedDates={{
  '0': {disabled: true, disableTouchEvent: true}
}}

This prop would disable Sundays.