Allow specifying config object for Command constructor

might as well re-use config if available..not sure how often this will
happen in practice

tests/commands/test_base.py

@@ -15,6 +15,7 @@ class TestCommand(TestCase):
         cmd = base.Command()
         self.assertEqual(cmd.name, 'wutta')
         self.assertIn('setup', cmd.subcommands)
+        self.assertEqual(str(cmd), 'wutta')
 
     def test_sorted_subcommands(self):
         cmd = base.Command(subcommands={'foo': 'FooSubcommand',