function externalLinks() { 
 if (!document.getElementsByTagName) return;
 var anchors = document.getElementsByTagName('a');
 for (var i=0; i<anchors.length; i++) {
   var anchor = anchors[i];
   if (anchor.getAttribute('href') && anchor.getAttribute('rel') == "external") 
     anchor.target = '_blank';
 }
}
window.onload = externalLinks;