Highlight active item in the main menu (#184)

docs/_includes/nav/collection_nav_group_item.html

@@ -1 +1,5 @@
-<li class="navListItem"><a class="navItem" href="{{ groupitem.url | absolute_url }}">{{ groupitem.title }}</a></li>
+<li class="navListItem">
+    <a class="navItem {% if page.url == groupitem.url %} navItemActive {% endif %}" href="{{ groupitem.url | absolute_url }}">
+        {{ groupitem.title }}
+    </a>
+</li>