Define first day of week for localisation uses.

To use, set a new value before initialising a calendar object.

    Calendar.firstDayOfWeek = 1; // monday
    new Calendar();

Inspired by https://github.com/Gargaj/calendarview/
Define weekends as `Calendar.weekendDays` so they may be adjusted.
Add a callback for each selectable date shown.  If `disableDateCallback`
returns false then corresponding cell is unreactive.
Merge branch 'weekdays'
Merge branch 'filter'