feat: make code type insensitive (close: #347)

Author: ulivz

lib/markdown/highlight.js

@@ -17,6 +17,7 @@ module.exports = (str, lang) => {
   if (!lang) {
     return wrap(str, 'text')
   }
+  lang = lang.toLowerCase()
   const rawLang = lang
   if (lang === 'vue' || lang === 'html') {
     lang = 'markup'