Kirby: Adding icon fonts to the navigation

about.txt

title: About
----
description: My awesome about site
----
icon: 

menu.php

<nav class="menu" role="navigation">
   <ul>
      <?php foreach($pages->visible() AS $p): ?>
      <li>
         <a class="<?php echo $p->uid() ?><?php echo ($p->isOpen()) ? ' active' : '' ?>" href="<?php echo $p->url() ?>">
            <?php echo html($p->title()) ?>
            <span data-icon="<?php echo $p->icon() ?>" aria-hidden="true"></span>
         </a>
      </li>
      <?php endforeach ?>
   </ul>
</nav>