[UX] Markdown supporting for concisely report

[bibochip02]

Thank you for great undercover feature. Otherwise I wonder could we support a summary report with Markdown format supporting?

Reasons:

1. With line-by-line comment style in reviewdog, It will make the pull request unreadable.
2. reviewdog is good, but It will make confuse when we have many commits be pushed.
3. Markdown with a last report is easier to follow and review/fix review code.

[grodowski]

Hey @bibochip02! I remember https://github.com/aki77/undercover-checkstyle achieved what you might be looking for, but using checkstyle instead of Markdown. There might be a possibility to build this kind of formatting in undercover itself, but I'm afraid I won't be able to support this anytime soon. Happy to help and brainstorm a potential PR with you if that's interesting!