Add syntax highlighting for code blocks

We might consider extracting this into an extension, but TextFormatter
does syntax highlighting for code blocks by default in live previews
anyway.

closes #248

framework/core/less/forum/Post.less

@@ -114,6 +114,11 @@
     color: #666;
     font-size: 90%;
     border-radius: @border-radius;
+
+    .hljs {
+      padding: 0;
+      background: none;
+    }
   }
   h1 {
     font-size: 160%;