Markdown: Workaround for incorrect highlighting due to double wrap hook (#2719)

The hook that highlights code blocks in markdown code was unable to handle code blocks that were highlighted already. The hook can now handle any existing markup in markdown code blocks.

Author: RunDevelopment

components/prism-markdown.js

@@ -347,8 +347,10 @@
 				});
 			}
 		} else {
-			// reverse Prism.util.encode
-			var code = env.content.replace(/&lt;/g, '<').replace(/&amp;/g, '&');
+			// get the textContent of the given env HTML
+			var tempContainer = document.createElement('div');
+			tempContainer.innerHTML = env.content;
+			var code = tempContainer.textContent;
 
 			env.content = Prism.highlight(code, grammar, codeLang);
 		}