title: Allow argparse nargs="+" to accept comma-separated input with choices
body: <p>I want to use argparse's {choices} parameter, but allow the user to input any number of items from choices. For example if choices is [1,2,3], I would like the following to be valid:</p>
  
  <pre><code>--arg 1
  --arg 1,2
  --arg 1,3
  </code></pre>
  
  <p>etc.</p>
  
  <p>However it seems like choices doesn't accept a comma-separated input when using nargs="+". Is there any way around this? I still want to enforce that the passed in options are within the set of choices that I defined, to error-check for weird inputs.</p>