function externalLinks() {
    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") == "external") { anchor.target = "_blank"; }
    }
}
externalLinks();