Add weekday name format for DateTimes

[tippenein]

I wanted a weekday name rather than the index. These changes allow a format string such as "AA, MMM D" and return something like "Sunday, April 1". ("A" for "Sun")

Let me know if I missed something already in place to handle this formatting!

[CLAassistant]

All committers have signed the CLA.

[safareli]

LGTM, thanks!
@cryogenian what you think?

[garyb]

This'll need releasing as a breaking change since it introduces new constructors. Looks good though!

[stefanholzmueller]

I like this functionality, too. Maybe the format tokens should be ddd and dddd though? See https://momentjs.com/docs/#/displaying/

[tippenein]

@stefanholzmueller good call. I was going off strftime but seeing this momentjs table makes it pretty clear that was misguided.

[cryogenian]

Thanks!