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")) {
      if (anchor.getAttribute("rel") == "external") {
        anchor.target = "_blank";
      }
    }
  }
}

addEvent(window, 'load', externalLinks);