window.onload = externalLinks;

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";
  }
}