feat($markdown): pass rest options to instantiate markdown-it

Close: #1040

e.g.

  markdown: {
    config: md => {
      md.set({linkify: true});
    }
  }

shortcut:

  markdown: {
    linkify: true
  },

Author: ulivz

lib/markdown/index.js

@@ -17,10 +17,10 @@ module.exports = ({ markdown = {}} = {}) => {
   // allow user config slugify
   const slugify = markdown.slugify || _slugify
 
-  const md = require('markdown-it')({
+  const md = require('markdown-it')(Object.assign({
     html: true,
     highlight
-  })
+  }, markdown))
     // custom plugins
     .use(component)
     .use(highlightLines)