First day option

[elliscwc]

this PR fixes #266
it allows setting first day of week from Sunday (0) to Saturday (6), I have tested with moment locale option changed, no issue so far

[peacechen]

Thanks @elliscwc for adding this much requested feature.

There are currently users who rely on the startFromMonday. Please add that back, but internally use firstDay set appropriately. That way all the calculations use your new generalized code.

The previous code used 1-based days of the week, whereas your code is 0-based. Does that have any ramifications with ISO dates vs non-ISO dates?

[elliscwc]

Ok, I added back the startFromMonday, will take it as consideration first only then firstDay prop.

Since the first item of WEEKDAYS array order in Utils is from 'Sun' or 0, it should not have issue. Also if it is '0', the starting index will use firstWeekDay.

However I have changed the getWeekdaysOrder to return ISO weekdays for customDayHeaderStyles.

[peacechen]

Thanks @elliscwc. This looks good. I'll merge after testing it

[peacechen]

Published in 7.1.1