Add a sensible way of specifying selectedDate on TinyDatePicker creation

[jarmo]

Currently TinyDatePicker constructor does not seem to have an option to specify date upon initialization, which might become handy sometimes. It would be a good to have a way to do that since current option is too cumbersome:

var datepicker = TinyDatePicker(el, ..)
datepicker.setState({selectedDate: SPECIFIED_DATE})