Add a way to unselect date

[martinpelli]

Is your feature request related to a problem? Please describe. I have two date picker timelines, and when I select the first one I am rebuilding the widget to unselect the second one, because second one depends on first one. Problem is that rebuilding second date picker with initialSelectedDate with null is not taking any effect. I achieved the result rebuilding that property with null and with datePickerController.animateToSelection() but is raising an exception because currentDate is null but is actually what I need, currentDate in null.