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;