title: Interactively select multiple items
body: <p>How can I let the user select multiple items from a list instead of just one? Like in the C-x b menu in helm.</p>
  
  <p>Currently I can only select a single item instead of getting a complete list:</p>
  
  <pre><code>(defun test-main ()
    (interactive)
    (let ((choice (completing-read "Select: " '("item1 item2 item3"))))
      (message choice)))
  </code></pre>