Insert code tag for server-side syntax highlighting

Inserts a code tag into Pygments output with the language-info that is present when using client-side highlighting (useful for CSS hooks)

```html
<code class="language-go" data-lang="go">
```

closes #1490

1 files changed, 11 insertions, 2 deletions

diff --git a/helpers/pygments.go b/helpers/pygments.go
index 29a5ec54f..17c30ee0a 100644
--- a/helpers/pygments.go
+++ b/helpers/pygments.go
@@ -111,14 +111,23 @@ func Highlight(code, lang, optsStr string) string {
 		return code
 	}
 
+	str := out.String()
+
+	// inject code tag into Pygments output
+	if lang != "" && strings.Contains(str, "<pre>") {
+		codeTag := fmt.Sprintf(`<pre><code class="language-%s" data-lang="%s">`, lang, lang)
+		str = strings.Replace(str, "<pre>", codeTag, 1)
+		str = strings.Replace(str, "</pre>", "</code></pre>", 1)
+	}
+
 	if cachefile != "" {
 		// Write cache file
-		if err := WriteToDisk(cachefile, bytes.NewReader(out.Bytes()), fs); err != nil {
+		if err := WriteToDisk(cachefile, strings.NewReader(str), fs); err != nil {
 			jww.ERROR.Print(stderr.String())
 		}
 	}
 
-	return out.String()
+	return str
 }
 
 var pygmentsKeywords = make(map[string]bool)