Use optional outputFormat for single formatting, allow moment JS to hand...

...le multiple input formats


This is intended to address the following issue:
https://github.com/dbushell/Pikaday/issues/142

There can be only one output format, regardless of how many input formats you want to handle (using MomentJS), so this patch allows explicitly specifying which output format you want to use. There's some sanity-checking in terms of which values to use by default if the "format" option is specified (and singular), but the "outputFormat" option is not specified.