title: Update argparse namespace with other namespace/dictionary
body: <p>Let's say I have two argparse namespaces</p>
  <pre><code>parser1 = argparse.ArgumentParser()
  parser1.add_argument('--name', type=str, required=False, default='John')
  
  args1 = parser1.parse_args()
  
  parser2 = argparse.ArgumentParser(argument_default=argparse.SUPPRESS)
  parser2.add_argument('--name', type=str, required=False)
  
  args2 = parser2.parse_args()
  </code></pre>
  <p>How can I update <code>args1</code> with <code>args2</code>? I am aware of updating dicts, i.e.</p>
  <pre><code>dict = {'name': 'Pete'}
  dict.update(**vars(args2))
  </code></pre>
  <p>This should work I think (not tested), but can you also update an argparse namespace with another namespace? I would be fine with converting <code>args2</code> to a dict to be able to update.</p>