Define management commands via function

[Olivier Le Thanh Duong]

Creating a new management command currently require a lot of boilerplate to remember, a small part of it is having to subclass a command. This is often overkill when you just have a function and don't need complex arguments parsing.

So I'm proposing to allow using function for Command.

I'm attaching an idea of an implementation but I'm not really set on it. This would allow to write:

    from django.core.management.base import LabelCommand

    def print_label(label):
        "Help message"
        print label

    Command = LabelCommand.from_func(print_label, can_import_settings=False)