function setExternalLinks() {
    if ( !document.getElementsByTagName ) {
        return null;
    }
 
    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.setAttribute( "target", "_blank" );
        }
    }
}

window.onload = setExternalLinks;