title: How to forbid two conflicting options
body: <p>Is there a way to specify to Python's ArgumentParser that two optional flags are conflicting?</p>
  
  <pre><code>arg_parser.add_argument('-c', '--clean', action='store_true')
  arg_parser.add_argument('-d', '--dirty', action='store_true')
  </code></pre>
  
  <p>I want the user to be able to specify either none of those, or only one.</p>
  
  <p>Is it achievable without further conditions?</p>