Use source/CNAME (if it exists) to set _config.yml url property

Rakefile

@@ -337,7 +337,11 @@ task :setup_github_pages, :repo do |t, args|
       end
     end
   end
-  url = "http://#{user}.github.io"
+  url = if File.exists?('source/CNAME')
+    "http://#{IO.read('source/CNAME').strip}"
+  else
+    "http://#{user}.github.io"
+  end
   url += "/#{project}" unless project == ''
   jekyll_config = IO.read('_config.yml')
   jekyll_config.sub!(/^url:.*$/, "url: #{url}")