Rework plugin to work a bit more cleanly. Add metadata and print blurb at bottom of posts.

_plugins/markdown_and_sign.rb

@@ -1,11 +1,14 @@
+# Adds metadata to the post object for linking to the source page.
+Jekyll::Hooks.register :posts, :pre_render do |post, payload|
+  if post.path =~ /\.md$/
+    post.data['md_file'] = "/#{post.data['category']}/#{post.basename}"
+  end
+end
+
 # Copy all of the source markdown files directly into the destination.
 Jekyll::Hooks.register :site, :post_write do |site|
   dest = site.dest
-  categories = site.config['category_metadata'].each_key do |category|
-    dir = Dir.new("#{site.source}/_posts/#{category}")
-    dir.each do |filename|
-      next unless filename =~ /\.md$/
-      FileUtils.cp("#{dir.path}/#{filename}", "#{dest}/#{category}/")
-    end
+  site.posts.docs.select{|p| p.path =~ /\.md$/}.each do |post|
+    FileUtils.cp(post.path, "#{dest}/#{post.data['category']}/#{post.basename}")
   end
 end