Allow passing functools.partial as entries in the subgroups dict

[lebrice]

Partially solves #195

• Allows passing functools.partial objects in the subgroups dict

• lambda expressions aren't allowed as the default_factory yet!

  • This is because we need to know what the default key is for that choice, and it isn't in the subgroup dict.
  • This is also because we can't inspect the default values for the fields without actually calling the factory.

  Therefore, you should use functools.partial for now.

Also somewhat related to #191, in that it makes this kind of thing possible again.

Future work: Integrating the Partials feature of #156 with this. This should work out-of-the box, since those are dataclasses that also inherit from functools.partial, but we'll see! :)