window.onload = externalLinks;

function externalLinks() {
  if(!document.getElementsByTagName) return;
  var links,a,i;
  links = document.getElementsByTagName("a");
  for(i=0;i<links.length;i++) {
    a = links[i];
    if(a.getAttribute("rel") && a.getAttribute("rel").indexOf("external") > -1) {
      a.onclick = function() {
        window.open(this.href);
        return false;
      }
    }
  }
}