initiate date picker from predefined list

[3ace]

this PR will add new constructor to display a predefined list of date.

example:

DatePicker.fromList(
  [
    DateTime(2020, 1, 1),
    DateTime(2020, 2, 1),
    DateTime(2020, 3, 1),
    DateTime(2020, 4, 1),
    DateTime(2020, 5, 1),
    DateTime(2020, 6, 1),
    DateTime(2020, 7, 1),
    DateTime(2020, 8, 1),
    DateTime(2020, 9, 1),
    DateTime(2020, 10, 1),
    DateTime(2020, 11, 1),
    DateTime(2020, 12, 1),
  ],
  width: 60,
  height: 80,
  initialSelectedDate: DateTime(2020, 4, 1),
  selectionColor: Colors.black,
  selectedTextColor: Colors.white,
)