Add extra_body block to index.html

This enables theme users to add HTML snippets to all the pages in the blog (like an analytics snippet).

templates/index.html

@@ -116,6 +116,8 @@
     {% endblock footer %}
 
 </div>
+{%- block extra_body %}
+{% endblock extra_body -%}
 </body>
 
 </html>