Optional parameter in format functions to override the instance's locale

[butterflyhug]

Format functions [1] should accept an optional locale key. If provided, the specified locale overrides the one associated with the instance itself, for just one formatting operation.

Related to #2. Could be a port of moment#1621. See also #13, we may want an optional options hash with timezone and locale as possible options.

[1] For this purpose, "format functions" are frozenMoment.format(), frozenMoment.calendar(), and anything else sensible in the "Display" section of the docs)

[benesch]

👍

Man, the developers of this project are responsive.