config: Print usage if no subcommand is given.

oxrlib/config.py

@@ -40,6 +40,9 @@ class Configuration:
             if read_path != expected_path:
                 self.error("failed to read configuration file {!r}".format(expected_path))
 
+        if not hasattr(self.args, 'command'):
+            argparser.print_help()
+            exit(2)
         try:
             post_hook = getattr(self, '_post_hook_' + self.args.command)
         except AttributeError: