Add rel="noopener noreferrer" to external links

Background:
https://medium.com/@jitbit/target-blank-the-most-underestimated-vulnerab
ility-ever-96e328301f4c#.hduwdbjlb

core/modules/savers/download.js

@@ -34,6 +34,7 @@ DownloadSaver.prototype.save = function(text,method,callback,options) {
 	// Set up the link
 	var link = document.createElement("a");
 	link.setAttribute("target","_blank");
+	link.setAttribute("rel","noopener noreferrer");
 	if(Blob !== undefined) {
 		var blob = new Blob([text], {type: "text/html"});
 		link.setAttribute("href", URL.createObjectURL(blob));