title: Set default value with argparse and same dest attribute
body: <p>here is a simple example.</p>
  
  <pre><code>import argparse
  parser = argparse.ArgumentParser()
  parser.add_argument('-a', action = 'append_const', dest = 'letter', const = 'a')
  parser.add_argument('-b', action = 'append_const', dest = 'letter', const = 'b')
  parser.set_defaults(letter = ['a', 'b'])
  print(parser.parse_args('-a'.split()))
  print(parser.parse_args('-b'.split()))
  print(parser.parse_args(''.split()))
  </code></pre>
  
  <p>Results :</p>
  
  <pre><code>Namespace(letter=['a', 'b', 'a'])
  Namespace(letter=['a', 'b', 'b'])
  Namespace(letter=['a', 'b'])
  </code></pre>
  
  <p>Without the set_defaults line, results are :</p>
  
  <pre><code>Namespace(letter=['a'])
  Namespace(letter=['b'])
  Namespace(letter=None)
  </code></pre>
  
  <p>How is it possible to configure argparse for such results ?</p>
  
  <pre><code>Namespace(letter=['a'])
  Namespace(letter=['b'])
  Namespace(letter=['a', 'b'])
  </code></pre>