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.getAttribute("rel") == "external nofollow" ||
      anchor.getAttribute("rel") == "nofollow external" )
      )
    anchor.target = "_blank";
  }
}


if(window.addEventListener) {
  window.addEventListener('load', externalLinks, false);
} else if(window.attachEvent) {
  window.attachEvent('onload', externalLinks);
}