message: Update main.py
  Add 'train_dir' as command.
diff: @@ -225,7 +225,7 @@ def freeze_graph(load_file):
  parser = argparse.ArgumentParser()
  -argh.add_commands(parser, [gtp, bootstrap, train, freeze_graph,
  +argh.add_commands(parser, [gtp, bootstrap, train, train_dir, freeze_graph,
  selfplay, evaluate, validate, convert])
  if __name__ == '__main__':
  