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";
    }
  }
}

function windowOnLoad() {
  externalLinks();
}

window.onload = windowOnLoad;