More quarter support (add/subtract quarters; startOf/endOf quarter)

[andrei-cocorean]

This should solve part of #1500.

[ichernev]

That looks pretty good! Don't you also need a quarter setter (pass value to the getter and handle it).

[andrei-cocorean]

I don't need the setter, but I can add it. I think it should be equivalent to:

.add('month', (newQuarter - currentQuarter) * 3)

e.g.

moment([2014, 1, 10]).quarter(2).format();  // => 2014-05-10

I'll also take a look at parsing the quarter from a string.

[icambron]

Yeah, I think a setter is important, since 'Q' being in the unitAliases list sort of informally implies you can do moment.set('Q', 3) (which would call moment.quarter(3)). It will also nicely round out the feature.

[andrei-cocorean]

I added the setter and the parsing token.

[icambron]

Awesome. Thanks for working on this!