Add markdown-it-attrs plugin for custom headers anchors

Syntax: # Header {#header-id}

docs/.vuepress/config.js

@@ -48,7 +48,10 @@ module.exports = {
 		code: {
 			lineNumbers: false
 		},
-		linkify: true
+		linkify: true,
+	},
+	extendsMarkdown(md) {
+		md.use(require('markdown-it-attrs')); // to use custom headers anchors
 	},
 	plugins: [
 		'@vuepress/plugin-search',

docs/package.json

@@ -2,6 +2,7 @@
   "devDependencies": {
     "@vuepress/plugin-search": "^2.0.0-beta.38",
     "glob": "^7.2.0",
+    "markdown-it-attrs": "^4.1.3",
     "vuepress": "^2.0.0-beta.38",
     "vuepress-plugin-copy-code2": "^2.0.0-beta.36"
   },