Enhancement: Linking to next / previous model

To link from one model to another within a template ("pagination") it might be helpful to have linked models using `model.next` and `model.previous`.

Example:

    <nav>
      <ul>
        {% if page.model.previous %}
        <li><a href="{{page.model.previous.mdl_url}}">PREVIOUS</a></li>
        {% else %}
        <li>&nbsp;</li>
        {% endif %}
        {% if page.model.next %}
        <li><a href="{{page.model.next.mdl_url}}">NEXT</a></li>
        {% else %}
        <li>&nbsp;</li>
        {% endif %}
      </ul>
    </nav>