How to provide missing value lookup?

[Troy Melhase]

Hi,

I'd like to have json templates where missing values can be found via
a function call. Reading through the source, I'm not quite sure what
the best approach to this would be, but it seems like
setting`undefined_str` to a function and modifying `_Undefined()` to
check for a callable would be the best approach. Yes, no, maybe,
something else?


I'm using the js version, and I'm happy to write and submit the
patch. Thanks,

troy