Add support for fallback

[Frozenfire92]

For example, if I set allowEmpty in my environment.js I would also like to be able to specify a default instead of nothing. For example I could render -- where there is no date (instead of the Invalid Date message or nothing)

[Frozenfire92]

From a quick investigation, it looks like this is the right place to add it? If so I'd be happy to submit a PR for this

[jasonmit]

@Frozenfire92 I am fine with adding fallback support