Passing function to `format` without also passing `parser` results in errors

[mryellow]

Errors trying to do .match() on a function rather than a string.

https://github.com/sumcumo/vue-datepicker/blob/928a999f197d2047e73b5b83b14a248934a46a74/src/utils/DateUtils.js#L13-L21

Passing the undocumented parser property results in bypassing this codepath and no longer expecting format to always be a string:

https://github.com/sumcumo/vue-datepicker/blob/928a999f197d2047e73b5b83b14a248934a46a74/src/utils/DateUtils.js#L343-L345