Add tabsets in ToC (closes #1336)

[atusy]

This PR aims to solve #1336. Currently, I find two problems that needs someone's help

• [ ] Support navigation to inactive tabs
  • Otherwise, users have to click the ToC twice. Evaluating anchor.click() twice does not help.
• [ ] Support navigation to tabs with duplicated names

Example Rmd

---
title: "ToC and Tabset"
output:
  html_document:
    toc: true
    toc_float: true
---

# A {.tabset}


## tab A-1

content A-1

## tab A-2

content A-2

# B {.tabset}

## tab B-1

content B-1

## tab B-2

content B-2

# B2 {.tabset}

## tab B-1

content B-1

## tab B-2

content B-2

[romanzenka]

This is something that is causing us a lot of grief right now, can I help with getting this in?

[atusy]

@romanzenka If you have any ideas to improve, feel free to take over my PR! I currently have no ideas to improve.