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 ;