function externalLinks() {
  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";
    }
  }
}

function init() {
  if (!document.getElementsByTagName) return;
  externalLinks();
}

window.onload = init;