window.onload = function() {
  if (document.getElementById('page')) {
    var elm = document.getElementById('page');
    add_target(elm);
  }
}

function add_target(elm) {
  var anchors = elm.getElementsByTagName("a");
  for (var i=0; i<anchors.length; i++) {
    if (anchors[i].className == "external") {
      anchors[i].target = "_blank";
    }
  }
}