Works in IE and Chrome:
function fixExternalLinks() {
var nodes = document.querySelectorAll('.wikiexternallink a');
for(var i=0; i< nodes.length; i++){
var link = nodes[i];
if (link.hasAttribute('target') == false) {
link.setAttribute('target', '_blank');
}
}
}
document.addEventListener("DOMContentLoaded", fixExternalLinks);