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