only load highlight.js if there's a 'pre code' block on page

index.html

@@ -408,7 +408,7 @@ function linkify( selector ) {
 					{ src: 'lib/js/classList.js', condition: function() { return !document.body.classList; } },
 					{ src: 'plugin/markdown/marked.js', condition: function() { return !!document.querySelector( '[data-markdown]' ); } },
 					{ src: 'plugin/markdown/markdown.js', condition: function() { return !!document.querySelector( '[data-markdown]' ); } },
-					{ src: 'plugin/highlight/highlight.js', async: true, callback: function() { hljs.initHighlightingOnLoad(); } },
+					{ src: 'plugin/highlight/highlight.js', async: true, condition: function() { return !!document.querySelector( 'pre code' ); }, callback: function() { hljs.initHighlightingOnLoad(); } },
 					{ src: 'plugin/zoom-js/zoom.js', async: true, condition: function() { return !!document.body.classList; } },
 					{ src: 'plugin/notes/notes.js', async: true, condition: function() { return !!document.body.classList; } }
 				]