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.target = "_blank";
  }
}
window.onload = externallinks;