config doesn't expose separator as an argument

[Ching-Hwa Yu]

This is to have something other than "_" for python files. The use case is to allow underscores in keys.

Something like:

def config(
    *configs: Iterable,
    prefix: str = "",
    remove_level: int = 1,
    separator: str = "__",
    lowercase_keys: bool = False,
    ignore_missing_paths: bool = False
) -> ConfigurationSet:

instances.append(config_from_env(*params, separator=separator, lowercase_keys=lowercase_keys))

instances.append(config_from_python(*params, separator=separator, lowercase_keys=lowercase_keys))