[New] `DayPickerSingleDateController`: add `allowUnselect` prop that allows controlling whether unselecting selected days is possible

[Docs] examples: `DateRangePickerWrapper`/`SingleDatePickerWrapper`: Add open direction as default props

This reverts commit 3e66eba4, reversing
changes made to 8c0ba988.

Fixes #818.

Added a regression unit test for the miss of passing onClose from SingleDatePicker to DayPickerSingleDateController as well as examples of using onClose with SingleDatePicker and DateRangePicker.

Adjust DayPickerRangeController iniitalVisibleMonth prop to match that of the DayPickerSingleDateController

Also fix the examples wrappers

The per-method packages are zero-dependency modules, so they often
include a bunch of extra code compared to importing from lodash/foo. We
can save some bundle size by using these partial includes instead.

Additionally, if I recall correctly, the per-method packages are all
deprecated and no longer updated, so this is the way to go for lodash
moving forward.

- Adds screen reader text to the input to describe how to enter the calendar
- Adds arrow navigation to the calendar itself
- Adds a keyboard shortcuts panel that pops up on top of the calendar
- Adds screen reader text to each CalendarDay