Stop opening external links in new tabs

We accept that this may be desired by forum owners and will offer an
extension to enable this feature. By default, we will not make any
assumptions and simply adopt the web's and browsers' default behavior.

Fixes #859.

src/Formatter/Formatter.php

@@ -139,7 +139,6 @@ class Formatter
         $dom = $configurator->tags['URL']->template->asDOM();
 
         foreach ($dom->getElementsByTagName('a') as $a) {
-            $a->setAttribute('target', '_blank');
             $a->setAttribute('rel', 'nofollow ugc');
         }