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';
    }
}
window.onload = externalLinks;