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;