Add CLI option --check to check whether the TOC is up to date

[akheron]

This is useful in CI to check whether TOC was updated or not when changing the headings in a markdown file.

[akheron]

I also documented the new option in .verb.md as instructed in the readme, but running verb changed README.md considerably (effectively broke it). Is there something I missed, or should I just not run verb and add the change to README.md by hand instead?

[joaopgrassi]

This would be great to have as part of CI checks, to see if a PR is lacking to update the TOC. Any chance we can have this, or can be achieve already in some other way? I checked but didn't find much.