title: argparse argument input as raw text or file
body: <p>Is there easy to get input using argparse whether it is raw text or file name based on prefix char, e.g.</p>
  <pre><code>python program.py -data @filename.txt
  python program.py -data 'raw text'
  </code></pre>
  <p>so the above programs should be equivalent if content of <code>filename.txt</code> is <code>raw text</code>.</p>