Provide a way to hightlight Sundays only

[jochenberger]

I'd like to use a different color for Sundays. Currently, there is the react-datepicker__day--weekend class, but no separate class for Sundays.
I could use .react-datepicker__day:nth-child(1) or .react-datepicker__day:nth-child(7), but that would mean that I need to know whether Sunday is the first or last day of the week. Also, it would probably cause issues when using DatePickers with different locales on one page.
One solution could be to add the weekday as a class to the day divs, like in react-datepicker__day--sunday. Another one would be adding a data attribute to the root element that denotes the first day of the week (like in data-first-day-of-week="sunday") or the index of the Sundays in the calendar (like in data-index-of-sunday="0").
I can create a PR when we decide how this should be addressed.

[jochenberger]

See also #514

[jochenberger]

@martijnrusschen: thoughts?

[stale]

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