feat($theme-default): support shortcut for sh and yml. (close: #1221)

Also, support lang tip for docker, dockerfile and makefile

packages/@vuepress/core/lib/app/style/config.styl

@@ -17,6 +17,6 @@ $MQMobileNarrow = 419px
 
 // code
 $lineNumbersWrapperWidth = 3.5rem
-$codeLang = js ts html md vue css sass scss less stylus go java c sh yaml py
+$codeLang = js ts html md vue css sass scss less stylus go java c sh yaml py docker dockerfile makefile
 
 @import '~@temp/palette.styl'

packages/@vuepress/markdown/lib/highlight.js

@@ -30,6 +30,12 @@ module.exports = (str, lang) => {
   if (lang === 'py') {
     lang = 'python'
   }
+  if (lang === 'sh') {
+    lang = 'bash'
+  }
+  if (lang === 'yml') {
+    lang = 'yaml'
+  }
   if (!prism.languages[lang]) {
     try {
       loadLanguages([lang])