Add support for markdown format

[ad-si]

E.g. as used for Scrut:
https://facebookincubator.github.io/scrut/docs/reference/formats/markdown-format/

[simonmichael]

Definitely, this would be good.

Also thanks for the link! I hadn’t seen scrut.

[ad-si]

FYI: I switched from Shelltest to Scrut here: https://github.com/ad-si/Woxi/tree/main/tests/cli, but surprisingly Shelltestrunner was faster! So I'd be happy to switch back if Shelltest supports a markdown format =D