TOC helper with Katex not works well

## Environment Info

Node version(`node -v`):
v9.4.0

Your site `_config.yml` (Optional):

Your theme `_config.yml` (Optional):

Hexo and Plugin version(`npm ls --depth 0`):

```
hexo-site@0.0.0 /Users/wafer/Documents/wafer-li.github.io
├── hexo@3.4.4
├── hexo-all-minifier@0.5.1
├── hexo-deployer-git@0.3.1
├── hexo-filter-plantuml@1.0.2
├── hexo-generator-archive@0.1.5
├── hexo-generator-atom-markdown-writer-meta@0.1.1
├── hexo-generator-category@0.1.3
├── hexo-generator-feed@1.2.2
├── hexo-generator-index@0.2.1
├── hexo-generator-searchdb@1.0.8
├── hexo-generator-seo-friendly-sitemap@0.0.21
├── hexo-generator-tag@0.2.0
├── hexo-renderer-ejs@0.3.1
├── hexo-renderer-markdown-it-plus@1.0.2
├── hexo-renderer-stylus@0.3.3
├── hexo-server@0.3.1
├── hexo-symbols-count-time@0.2.0
├── markdown-it-criticmarkup@0.0.1
└── markdown-it-task-lists@2.1.0
```

## For BUG

BUG description here: https://github.com/theme-next/hexo-theme-next/pull/32#issuecomment-359018694

I use `hexo-renderer-markdown-it-plus` as the renderer, and it uses `markdown-it` to parse markdown and `markdown-it-katex` to generate Katex HTML stuff under the hood.

## For question

## For feature request