fix($markdown): cannot highlight external python file (.py) correctly (close: #660)

Author: ulivz

lib/markdown/highlight.js

@@ -29,6 +29,9 @@ module.exports = (str, lang) => {
   if (lang === 'ts') {
     lang = 'typescript'
   }
+  if (lang === 'py') {
+    lang = 'python'
+  }
   if (!prism.languages[lang]) {
     try {
       loadLanguages([lang])