function anchor_target_blank() {
    if (!document.getElementsByTagName) {
        return;
    }
    var anchors = document.getElementsByTagName("a");
    for (var i = 0; i < anchors.length; i++) {
        var anchor = anchors[i];
        if (anchor.getAttribute("href") &&
            anchor.getAttribute("rel") &&
            anchor.getAttribute("rel").indexOf("external") >= 0) {
            anchor.target = "_blank";
        }
    }
}
Event.observe(window, 'load', function() {
    anchor_target_blank();
});