title: Argparse pattern where input filenames are given through a file or a list of filenames on the command line
body: <p>Does argparse support a pattern such as:</p>
  
  <pre><code>foo.py {-f list_of_filenames.txt|file [file ...]}
  </code></pre>
  
  <p>I achieve this at the moment with the following argparse definition:</p>
  
  <pre><code>parser = argparse.ArgumentParser()
  parser.add_argument("--file", "-f")
  parser.add_argument("files", nargs="*")
  </code></pre>
  
  <p>and performing the mutual exclusivity + required check in my code rather than letting argparse do the check. </p>
  
  <p>Can argparse handle a pattern of this sort?</p>