Table of contents generator

I recommend adding a table of contents generator feature. A placeholder like pandoc's --toc or MultiMarkdown's {{toc}} will be expanded into list of links nested by heading level.

Given the markdown:

# Markdown
--toc
# History
John Grubner created the Markdown language in 2004.
## Standardization
There is no clearly defined Markdown standard, apart from the original writeup and implementation by John Gruber, which some consider abandonware.
## CommonMark
From 2012, a group of people including Jeff Atwood launched what Atwood characterized as a standardization effort.
# Implementations
Implementations of Markdown are available for over a dozen programming languages.

The generated html will be

<h1 id="markdown">Markdown</h1>
<ul>
    <li>
        <a href="#history">History</a>
        <ul>
            <li><a href="#standardization">Standardization</a></li>
            <li><a href="#commonmark">CommonMark</a></li>
        </ul>
    </li>
    <li><a href="#implementations">Implementation</a></li>
</ul>
<h1 id="history">History</h1>
<p>John Grubner created the Markdown language in 2004.</p>
<h2 id="standardization">Standardization</h2>
<p>There is no clearly defined Markdown standard, apart from the original writeup and implementation by John Gruber, which some consider abandonware.</p>
<h2 id="commonmark">CommonMark</h2>
<p>From 2012, a group of people including Jeff Atwood launched what Atwood characterized as a standardization effort.</p>
<h1 id="implementations">Implementations</h1>
<p>Implementations of Markdown are available for over a dozen programming languages.</p>

Which looks like this:

Markdown

• History
  • Standardization
  • CommonMark
• Implementation

History

John Grubner created the Markdown language in 2004.

Standardization

There is no clearly defined Markdown standard, apart from the original writeup and implementation by John Gruber, which some consider abandonware.

CommonMark

From 2012, a group of people including Jeff Atwood launched what Atwood characterized as a standardization effort.

Implementations

Implementations of Markdown are available for over a dozen programming languages.

No files or tests are named in the issue; begin by locating Markdig’s Markdown parsing and HTML rendering entry points. Compare the requested placeholder behavior and sample output with existing heading-anchor handling, then define tests covering nested heading levels and verify the generated HTML matches the example.