title: How to set argument_default=argparse.SUPPRESS for subparsers?
body: <p>I don't want to get the unset options in the argument namespace. But <code>argparse.SUPPRESS</code> seems not pass on to subparsers.
  The following code <code>print(args)</code> got the result. Since sub_arg2 and sub_args3 is not set, how to  get ride of them in the namespace?</p>
  <blockquote>
  <p>Namespace(sub_arg1='1', sub_arg2=[], sub_arg3=None, subparser='sub1')</p>
  </blockquote>
  <pre class="lang-py prettyprint-override"><code>import argparse
  
  parser = argparse.ArgumentParser(argument_default=argparse.SUPPRESS)
  # parser.add_argument('main_arg1')
  # parser.add_argument('main_arg2', nargs='*')
  subparsers = parser.add_subparsers(dest='subparser')
  sparser = subparsers.add_parser('sub1')
  sparser.add_argument('sub_arg1')
  sparser.add_argument('sub_arg2', nargs='*')
  sparser.add_argument('sub_arg3', nargs='?')
  
  args = parser.parse_args('sub1 1'.split())
  print(args)
  </code></pre>