title: format string email to email-link
body: <p>I have a nullable-string email, and I would like to format the string as a mailto-link if it seems valid.</p>
  
  <p>like this:</p>
  
  <pre><code>&lt;a href="mailto:foo@foo.com"&gt;foo@foo.com&lt;/a&gt;
  </code></pre>
  
  <p>How is that done?</p>
  
  <p>/M</p>