extra_head template hook. This will allow plugins to add extra js/css more easily.

This commit sponsored by Moritz Berberich.  Thank you!

mediagoblin/templates/mediagoblin/base.html

@@ -36,7 +36,14 @@
       <script src="{{ request.staticdirect('/js/extlib/html5shiv.js') }}"></script>
     <![endif]-->
 
+    {# For clarification, the difference between the extra_head.html template
+     # and the extra_head template hook is that the former should be used by
+     # themes and the latter should be used by plugins.
+     # The reason is that only one thing can override extra_head.html...
+     # but multiple plugins can hook into the template hook.
+     #}
     {% include "mediagoblin/extra_head.html" %}
+    {% template_hook("extra_head") %}
 
     {% block mediagoblin_head %}
     {% endblock mediagoblin_head %}