title: Argparse with action='store_true' not working as expected
body: <p>The idea is to add a flag (<code>--slack</code>, or <code>-s</code>) when running the script, so that I don't have to comment out the <code>rep.post_report_to_slack()</code> method every time I don't want to use it. When I run:</p>
  
  <pre><code>$ python my_script.py --slack
  </code></pre>
  
  <p>I get the error:</p>
  
  <pre><code>my_script.py: error: unrecognized arguments: --slack
  </code></pre>
  
  <p>Here's the code:</p>
  
  <pre><code>def main():
      gc = Google_Connection()
      meetings = gc.meetings
  
      rep = Report(meetings)
  
      if args.slack:
          rep.post_report_to_slack()
          print('posted to slack')
  
  
  if __name__ == '__main__':
      import argparse
      parser = argparse.ArgumentParser()
      parser.add_argument('-s', '--slack', help='post to slack', 
          action='store_true')
      args = parser.parse_args()
      main()
  </code></pre>