Amend the existing rel attribute of links

...instead of overwriting. This will play more nicely with extensions.

Refs #859.

src/Formatter/Formatter.php

@@ -137,7 +137,8 @@ class Formatter
         $dom = $configurator->tags['URL']->template->asDOM();
 
         foreach ($dom->getElementsByTagName('a') as $a) {
-            $a->setAttribute('rel', 'nofollow ugc');
+            $rel = $a->getAttribute('rel');
+            $a->setAttribute('rel', "$rel nofollow ugc");
         }
 
         $dom->saveChanges();