feat: refine markdown api (#1337)

1. support env.loader
2. refine link behavior, every token is an instance of `Token`. its prototype should not be removed

Author: shigma

packages/@vuepress/markdown-loader/index.js

@@ -21,6 +21,7 @@ module.exports = function (src) {
   const isProd = process.env.NODE_ENV === 'production'
   const isServer = this.target === 'node'
   const options = getOptions(this)
+  const loader = Object.create(this)
   const { sourceDir } = options
   let { markdown } = options
   if (!markdown) {
@@ -71,6 +72,7 @@ module.exports = function (src) {
     data: { hoistedTags, links },
     dataBlockString
   } = markdown.render(content, {
+    loader,
     frontmatter: frontmatter.data,
     relPath: path.relative(sourceDir, file)
   })

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

@@ -66,8 +66,8 @@ module.exports = (md, externalAttrs) => {
     const routerLinks = md.$data.routerLinks || (md.$data.routerLinks = [])
     routerLinks.push(to)
 
-    return Object.assign({}, token, {
-      tag: 'router-link'
+    return Object.create(token, {
+      tag: { value: 'router-link' }
     })
   }