function openExternal(){
    if(!document.getElementsByTagName) return;
    var anchors = document.getElementsByTagName("a");
    for(var i = 0; i < anchors.length; i++){
        var thisAnchor = anchors[i];
        if(thisAnchor.getAttribute("href") && thisAnchor.getAttribute("rel") == "external"){
            thisAnchor.target = "_blank";
        }
    }
}

window.onload = openExternal;