// Add .external and target="_blank" to rel="external" links.
var Links = {
  fixExternal: function() {
    $$("a[rel=external]").each(function(element) {
      element.addClassName("external");
      element.setAttribute("target", "_blank");
    });
  }
}

document.observe("dom:loaded", function() {
  Links.fixExternal();
});