[Feature] Add --insert option (like in shell project)

[ekalinin]

See:

• [ BUG ] No --insert option in homebrew's gh-md-toc github-markdown-toc#105

[damemi]

Hi! Have there been any updates on this? It would be very helpful for our project

[knennigtri]

Agree. I originally found the shell tool and loved the --insert feature... but then I wanted to use --depth, which I needed this project for. It would be ideal if both had both features, this is a great quick tool!