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

Close: #1040

e.g.

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

shortcut:

  markdown: {
    linkify: true
  },

packages/@vuepress/markdown/index.js

@@ -25,17 +25,18 @@ const { parseHeaders, slugify: _slugify, logger, chalk } = require('@vuepress/sh
  * Create markdown by config.
  */
 
-module.exports = ({
-  slugify,
-  externalLinks,
-  anchor,
-  toc,
-  lineNumbers,
-  beforeInstantiate,
-  afterInstantiate
-} = {}) => {
+module.exports = (markdown = {}) => {
+  const {
+    externalLinks,
+    anchor,
+    toc,
+    lineNumbers,
+    beforeInstantiate,
+    afterInstantiate
+  } = markdown
+
   // allow user config slugify
-  slugify = slugify || _slugify
+  const slugify = markdown.slugify || _slugify
 
   // using chainedAPI
   const config = new Config()
@@ -110,7 +111,7 @@ module.exports = ({
 
   beforeInstantiate && beforeInstantiate(config)
 
-  const md = config.toMd()
+  const md = config.toMd(require('markdown-it'), markdown)
 
   afterInstantiate && afterInstantiate(md)
 

packages/@vuepress/markdown/package.json

@@ -21,7 +21,7 @@
     "@vuepress/shared-utils": "^1.0.0-alpha.24",
     "markdown-it": "^8.4.1",
     "markdown-it-anchor": "^5.0.2",
-    "markdown-it-chain": "^1.2.1",
+    "markdown-it-chain": "^1.3.0",
     "markdown-it-container": "^2.0.0",
     "markdown-it-emoji": "^1.4.0",
     "markdown-it-table-of-contents": "^0.4.0",

yarn.lock

@@ -5556,9 +5556,9 @@ markdown-it-anchor@^5.0.2:
   version "5.0.2"
   resolved "https://registry.yarnpkg.com/markdown-it-anchor/-/markdown-it-anchor-5.0.2.tgz#cdd917a05b7bf92fb736a6dae3385c6d0d0fa552"
 
-markdown-it-chain@^1.2.1:
-  version "1.2.1"
-  resolved "https://registry.yarnpkg.com/markdown-it-chain/-/markdown-it-chain-1.2.1.tgz#eb77a8dc06265d48d0b6c729170e391902db22cb"
+markdown-it-chain@^1.3.0:
+  version "1.3.0"
+  resolved "http://registry.npm.taobao.org/markdown-it-chain/download/markdown-it-chain-1.3.0.tgz#ccf6fe86c10266bafb4e547380dfd7f277cc17bc"
   dependencies:
     webpack-chain "^4.9.0"