add support for .format() on .duration()

[ralyodio]

I would like to be able to format duration as time:
moment.duration(3600, 'seconds')).format('HH:mm:ss');

output should produce 01:00:00

[benmonro]

👍

[ichernev]

Related to #1698

This is the non-i18n version of the proposal. It should support finding out the tokens, sorting them, figuring out where to overflow (in HH:mm:ss minutes overflow to hours, but hours don't overflow) assign values to all tokens and then display.

Problematic areas are zero tokens (are you ok with 00:00:01?): the function may be given array of formats or assume the tokens are sorted in the string and slice from the first non-zero token to the end.

[icambron]

Marking as a dupe of #1698.