Alternate Clock Source Support

[atogata]

Fix #2337

Motivation: Need to implement client-side time synchronization for other project.

Example:

moment().toISOString()
"2015-11-22T23:02:47.586Z"

moment.fn.now = function() { return 10000; };
() { return 10000; }

moment().toISOString()
"1970-01-01T00:00:10.000Z"

moment.fn.now = function() { return Date.now() * 2; };
() { return Date.now() * 2; }

moment().toISOString()
"2061-10-13T22:06:22.934Z"

[jokesterfr]

I like it, it's a clean way to customize the now() method.

[icambron]

👍

[ichernev]

Merged in 816245f