Page sub-navigation

Styling for sub-navigation elements.

~~~
<nav class="subnav">
    <h2>Subnav</h2>
    <ul>
        <li><a href="#section-1">Section 1</a></li>
        <li><a href="#section-2">Section 2</a></li>
    </ul>
</nav>
~~~

Also updating pages app to allow for auto-generated subnavigation.