Add functions to conditionally load the config from disk or return the const struct

[mistodon]

I find myself continually writing functions to parse the config in debug mode, and return the constant in release mode.

Would be good to build this in, maybe behind a feature.

[mistodon]

Done: https://docs.rs/config_struct/0.2.0/config_struct/struct.Options.html#structfield.generate_load_fns