Add rel="noopener noreferrer" to external links

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

plugins/tiddlywiki/powered-by-tiddlywiki/banner.tid

@@ -1,6 +1,6 @@
 title: $:/plugins/tiddlywiki/powered-by-tiddlywiki/banner
 tags: $:/tags/PageTemplate
 
-<a href="http://tiddlywiki.com" target="_blank">
+<a href="http://tiddlywiki.com" target="_blank" rel="noopener noreferrer">
 [img width="160px" class="tc-powered-by-tiddlywiki-banner" [$:/plugins/tiddlywiki/powered-by-tiddlywiki/powered-by-tiddlywiki.svg]]
 </a>

plugins/tiddlywiki/railroad/wrapper.js

@@ -97,6 +97,7 @@ RailroadWidget.prototype.patchLinks = function(node) {
 					if(child.attributes["data-tw-external"]) {
 						// External links are straightforward
 						child.setAttribute("target","_blank");
+						child.setAttribute("rel","noopener noreferrer");
 					} else {
 						// Each internal link gets its own onclick handler, capturing its own copy of target
 						(function(myTarget) {