Add menu link to ChaiScript_Extras

Author: lefticus

_includes/header.html

@@ -46,6 +46,9 @@
         <li {% if page.url == '/support.html'%} class='active' {% endif %}>
         <a href="/support.html">Support</a>
         </li>
+        <li>
+        <a href="https://github.com/ChaiScript/ChaiScript_Extras">Extras</a>
+        </li>
       </ul>
     </div><!-- /.navbar-collapse -->
   </div><!-- /.container-fluid -->