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

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