Add k and kk formatting tokens

[not-an-aardvark]

This fixes issue #2762.

The consensus from that issue seemed to be that the presence of the kk token should not modify the value of other tokens, so that is the behavior that is implemented here.

It should be noted that this decision introduces a bit of an inconsistency, given that the parsing behavior introduced here does change the day when an hour of 24 is present.

In other words,

moment('2016-04-09T24:00:00')
// yields a moment representing midnight at the start of April 10th, introduced in #1965

moment('2016-04-09T24:00:00').format('YYYY-MM-DD[T]kk:mm:ss')
// "2016-04-10T24:00:00" (different from the original format string that was parsed)

---

I'm not extremely familiar with the internal codebase, so please let me know if there's anything that I did incorrectly or that I should add.

[ichernev]

Looks good, thank you @not-an-aardvark ! Will merge in next release.

[ichernev]

Merged in 486a9db