Support external links in navigation

Author: brb3

_includes/navigation.html

@@ -6,7 +6,7 @@
       {% for page in pages %}
         {% if page.menu_group > 0 %}
         <li data-menu-group="{{ page.menu_group }}">
-          <a class="page-link" href="{{ page.url | prepend: site.baseurl }}">{{ page.title }}</a>
+          <a class="page-link" href="{% if page.external_url %}{{ page.external_url }}{% else %}{{ page.url | prepend: site.baseurl }}{% endif %}">{{ page.title }}</a>
         </li>
         {% endif %}
       {% endfor %}