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-link-new-window") || (anchor.getAttribute("class") == "external-link-new-window")))
    ) {
        anchor.target = "_blank";
        anchor.title = (anchor.title != "") ? anchor.title+" (öffnet ein neues Fenster)" : "in einem Neuen Fenster:";
      }
  }
}

window.onload = externalLinks;