Add categories plugin

Gemfile.lock

@@ -100,4 +100,4 @@ DEPENDENCIES
   uglifier
 
 BUNDLED WITH
-   1.11.2
+   1.12.5

_includes/post/categories.html

@@ -1,10 +1,10 @@
-{% capture category %}{% if post %}{{ post.categories | category_links | size }}{% else %}{{ page.categories | category_links | size }}{% endif %}{% endcapture %}
+{% capture category %}{% if post %}{{ post.categories | category_list | size }}{% else %}{{ page.categories | category_list | size }}{% endif %}{% endcapture %}
 {% unless category == '0' %}
 <span class="categories">
   {% if post %}
-    {{ post.categories | category_links }}
+    {{ post.categories | category_list }}
   {% else %}
-    {{ page.categories | category_links }}
+    {{ page.categories | category_list }}
   {% endif %}
 </span>
 {% endunless %}

_plugins/categories.rb

@@ -0,0 +1,15 @@
+module Jekyll
+  module Filters
+
+    # Outputs a list of categories as comma-separated <a> links. This is used
+    # to output the category list for each post on a category page.
+    #
+    #  +categories+ is the list of categories to format.
+    #
+    # Returns string
+    #
+    def category_list(categories)
+      categories.sort.join(', ')
+    end
+  end
+end