CommonMark linting

[handrews]

Investigate whether CommonMark syntax can be validated. As the use of markdown in descriptions is a MAY, validation should probably be opt-in and off by default.

[handrews]

As expected, no string is invalid CommonMark so it technically cannot be validated, but it can be linted, and at least one linter exists in Python as an active project (a second package is now abandoned).