Allow heading numbering to start counting at later Headings and not just Heading 1

[ketozhang]

Often times <h1> or Markdown # is reserved for titles. Titles should not have a heading number however jupyterlab-toc provides no option to start counting the heading at later headings like ##.

Desired Behavior

## Section

<h2>1. Section</h2>

Current Behavior

## Section

<h2>0.1 Section</h2>

[chm123]

Duplicate of #66

[ketozhang]

Thanks, closing