bug: sidebar is missing

**Version**
Version 0.2.2

**Describe the bug**
As what you can see in the [demo](https://buptsteve.github.io/markdown-it-vuese/example/demo.html). We lose the sidebar that generated by Vuepress.

**Expected behavior**
There should have a sidebar.

**Additional context**
https://github.com/vuejs/vuepress/blob/master/packages/%40vuepress/markdown/index.js
