Feature request: Expanding environment variables in config files

[awgroeneveld]

Coming from a Spring background, I'm missing the possibility of expansion of environment variables referenced from within configuration files, giving an even more flexible approach next to overriding properties which is in itself a great functionality of the config crate.

Using this proposed change, there is no requirement to fully-qualify environment variable names that override the config files.
Spring also has support for a default value.

Both elements could be implemented, where :- splits the environment variable from the default, e.g.:

database:
   host: ${DBHOST:-127.0.0.1}
   port: ${DBPORT:-27017}

Would you mind considering the proposal? I'd be glad to head from you.

There's a draft PR (#706) with a test which shows the effect.

[epage]

Looks like we have an issue for this already in #345.