Fix site to categorize and show categories automatically.

_plugins/auto_categories.rb

@@ -0,0 +1,9 @@
+# Very simple plugin that adds a subdirectory under _posts as a category
+
+def get_category_from_subdir(path)
+  path.sub(/^.*_posts\/(.*?)\/.*$/, "\\1")
+end
+
+Jekyll::Hooks.register :posts, :pre_render do |post, payload|
+  post.data['categories'] << get_category_from_subdir(post.path)
+end