Add custom date format to pages, add 'updated' field again

Reverts changes of c2a68cc where I accidentally removed support for 'updated' field, see comments of issue #164 for details.

.themes/classic/source/_layouts/post.html

@@ -9,7 +9,7 @@ single: true
   <footer>
     <p class="meta">
       {% include post/author.html %}
-      {% include post/date.html %}{{ time }}
+      {% include post/date.html %}{% if updated %}{{ updated }}{% else %}{{ time }}{% endif %}
       {% include post/categories.html %}
     </p>
     {% unless page.sharing == false %}