sub_commands should support a "simple" mode where a plain list of classes is supplied

[zyga]

For usability, I never remember exactly what to put in sub_commands (in what order, to be precise).

I'd like this to work:

class foo(Command):
    name = 'foo'
class bar(Command):
   name = 'bar'
class main(Command):
    sub_commands = (foo, bar)