Feature: Output in Markdown, to support GitHub Action Job Summaries

[tdwright]

Inspired by this PR for Coverage.py, it would be awesome to (have the option to) output a summary of the scan results as markdown, for use in a Job Summary.

As a visual aid, here's what it looks like when I do this for Coverage.py:

The table at the bottom is just markdown piped to $GITHUB_STEP_SUMMARY, but it makes the results much more accessible compared to having to download a zip of the HTML report.