Default value callback

Howdy!

I'm just evaluating this library for a couple of F# projects, and I was wondering if there's any way to provide a function (or `Action<_>`) as the default instead of a static value. This would allow me to pull in values from elsewhere, namely `IConfiguration` so that I get I get config file & env values as the fallback too.

Is anything like this currently possible?

Cheers!