Add file extensions back to non-helper _includes

_includes/comments.html

@@ -0,0 +1,14 @@
+{% if site.comments.provider and page.comments %}
+
+{% case site.comments.provider %}
+{% when "disqus" %}
+  {% include /comments-providers/disqus %}
+{% when "facebook" %}
+  {% include /comments-providers/facebook %}
+{% when "google-plus" %}
+  {% include /comments-providers/google-plus %}
+{% when "custom" %}
+  {% include /comments-providers/custom %}
+{% endcase %}
+
+{% endif %}