Add canonical link to post page

_includes/themes/ploeh/default.html

@@ -6,6 +6,7 @@
     <title>{{ page.title }}</title>
     {% if page.description %}<meta name="description" content="{{ page.description }}">{% endif %}
     <meta name="author" content="{{ site.author.name }}">
+    {% if page.layout == "post" %}<link rel="canonical" href="{{ site.production_url | default: site.url }}{{ page.url }}">{% endif %}
 
     <!-- Le HTML5 shim, for IE6-8 support of HTML elements -->
     <!--[if lt IE 9]>