add class with day of the week index

Just wondering if you'd consider added a class like `day-0` thru `day-6` (or `dow` or similar) to the [`extraClasses`](https://github.com/kylestetz/CLNDR/blob/master/src/clndr.js#L378) string?

I realize this is trivial to add in a template via the Moment.js date object—just thought it might save a step for people down the road.

For the record, my use case was to style weekends differently than weekdays...