Add :short_year to url_placeholders

lib/jekyll/post.rb

@@ -225,6 +225,7 @@ module Jekyll
         :i_month     => date.strftime("%m").to_i.to_s,
         :categories  => (categories || []).map { |c| c.to_s }.join('/'),
         :short_month => date.strftime("%b"),
+        :short_year  => date.strftime("%y"),
         :y_day       => date.strftime("%j"),
         :output_ext  => output_ext
       }