title: Replace _ wrapper with html <i> tag
body: <p>How to get with javascript regex this:</p>
  
  <pre><code>&gt; "Who are _you_?" said _the Caterpillar_. 
  &gt; -- &gt;
  &gt; "Who are &lt;i&gt;you&lt;/i&gt;?" said &lt;i&gt;the Caterpillar&lt;/i&gt;.
  </code></pre>