Add 404 page which fits the rest of the theme

* Noting that (as of 0.14) zola does not pass much of a context when
  rendering the 404 page, so things like current URL/path aren't
  available
* To work around this the `menu` macro has been split into `menu_for`
  which accepts a parameter for which item is active.
* The previous macro continues to work as is, forwarding the results of
  figuring out which item is active to the new `menu_for` macro
* The added 404 page will then use the `menu_for` macro and pass in a
  blank item

templates/index.html

@@ -45,7 +45,9 @@
             </div>
         </div>
 
+        {% block header_menu %}
         {{ menu_macros::menu(config=config, current_path=current_path) }}
+        {% endblock header_menu %}
     </header>
     {% endblock header %}