Background color customizations, scrollToMonthForDate, custom initializer for reference date & days before/after

Implemented the following customizations while keeping existing defaults:
- Added background color customizations for today and selected day.
- Added scrollToMonthForDate (scrolls month header to top of view instead of centering specified date in view).
- Added init method that accepts a reference date and how many days before and after to show, to determine how far backward or forward a user can scroll.