[highlightjs] Use new 'head'/'footer' hooks

- Create separate addon javascript file
- Register stylesheet and Javascript files

highlightjs/highlightjs.js

@@ -0,0 +1,6 @@
+hljs.initHighlightingOnLoad();
+
+document.addEventListener('postprocess_liveupdate', function () {
+	var blocks = document.querySelectorAll('pre code:not(.hljs)');
+	Array.prototype.forEach.call(blocks, hljs.highlightBlock);
+});