Allow to use it inside Python scripts with declarative sequence of Groups

[cuducos]

For example, having this declarative structure: https://github.com/cuducos/bot-followers/blob/64eb67a56a85b0ba00544882c2d95f603aa88636/env.py#L60-L129

We should be able to write a Python-like to start the CLI:

parser = Parser.from_object(settings)  # settings is a Python object (e.g.: a tuple of createnv.parser.Group instances)
Generator(Path(".env"), parser, overwrite, use_default)()