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";
      anchor.title = (anchor.title != "") ? anchor.title+" (opens in a new window)" : "opens in a new window";
      anchor.className = (anchor.className != '') ? anchor.className+' external' : 'external';
    }
  }
}

window.onload = externalLinks;