Optionally add indentation to maps

This makes them easier to debug by showing the structure. Sample output
when $spaces is set to 4 spaces:

<div class='map'>
<ul>
    <li>
    <a href="../alpha" class="mapparent">alpha</a>
        <ul>
        <li>
        <a href="../alpha/1" class="mapitem">1</a>
        </li>
        </ul>
    </li>
    <li>
    <a href="../beta" class="mapitem">beta</a>
    </li>
</ul>
</div>