Add a get_user_permissions management command

[synw]

Hi, your module gave me the idea of inspecting user permissions for my django-introspection module.

I made a get_user_permissions management command that outputs the permissions of a given user.

I made a pull request to see if you are interested, otherwise I'll put it in my module. Note that this would add a dependency on django-introspection, which may not be what you want.