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/404.html

@@ -0,0 +1,16 @@
+{% extends "index.html" %}
+
+{% block title %}
+404
+{% endblock title %}
+
+{% block header_menu %}
+{{ menu_macros::menu_for(config=config, current_item="") }}
+{% endblock header_menu %}
+
+{% block content %}
+<div class="post">
+  <h1 class="post-title">Lost?</h1>
+  <p>This page does not exist.</p>
+</div>
+{% endblock content %}