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;