function linkFix() { 
  if (!document.getElementsByTagName) return;

  var hrefs = document.getElementsByTagName("a");
  for (var i=0; i<hrefs.length; i++) {
    var href = hrefs[i];
    if (href.getAttribute("href") && href.getAttribute("rel") == "external") {
      href.target = "_blank";
    }
  }
}

window.onload = linkFix;