Yet more tweaks to the highlight.js integration

Now we check that the selected language is supported before we invoke
highlight.js; left to its own devices, it crashes…

plugins/tiddlywiki/highlight/TypeMappings.multids

@@ -1,6 +1,5 @@
 title: $:/config/HighlightPlugin/TypeMappings/
 
-text/plain:
 application/javascript: javascript
 application/json: json
 text/css: css

plugins/tiddlywiki/highlight/highlightblock.js

@@ -27,7 +27,7 @@ CodeBlockWidget.prototype.postRender = function() {
 	if(tiddler) {
 		language = tiddler.fields.text || "";
 	}
-	if(language) {
+	if(language && hljs.listLanguages().indexOf(language) !== -1) {
 		domNode.className = language.toLowerCase() + " hljs";
 		if($tw.browser && !domNode.isTiddlyWikiFakeDom) {
 			hljs.highlightBlock(domNode);