manifest.json

{
   "content_scripts": [ {
      "js": [ "newtab.js" ],
      "matches": [ "*://*/*" ]
   } ],
   "description": "Always open external links in a new tab.",
   "manifest_version": 2,
   "name": "Open External Links in New Tab",
   "update_url": "https://clients2.google.com/service/update2/crx",
   "version": "1.0"
}

newtab.js

function isExternalLink(anchor, host) {
  var isHrefInternal = anchor.href.indexOf(host) != -1;
  if (isHrefInternal) {
    return false;
  }

  if (anchor.target != "" && anchor.target != "#") {
    return false;
  }

  return true;
}

(function() {
  var host = location.hostname;
  document.addEventListener("click", function(event) {
    var elem = event.target || event.srcElement;
    while (elem) {
      if (!(elem instanceof HTMLAnchorElement)) {
        elem = elem.parentNode;
        continue;
      }

      if (isExternalLink(elem, host)) {
        // https://blog.jxck.io/entries/2016-06-12/noopener.html
        elem.setAttribute("target", "_blank");
        elem.setAttribute("rel", "noopener noreferrer");
      }
      break;
    }
  });
})();