More quarter support

[philfreo]

I see that there's some support for quarters.

It would be nice to also support this:

moment().startOf('quarter')

Also would be nice to be able to parse a string in a format like 'YYYY-Q', but I don't see any quarter options listed on http://momentjs.com/docs/#/parsing/string-format/

[icambron]

That's right; quarter is only supported as a getter and format token for now. We'd accept a pull request for parsing and startOf/endOf if it was small and simple enough, but I don't think it's priority for Iskren or I to implement.

[icambron]

This is fixed by #1586 and is in the new release.