Add pre-commit check to ensure HTML fragments with MD uses proper attributes

[aarnq]

The problem

With the markdown in HTML plugin it is required to set attributes on nested HTML elements else they will not be rendered, which is an easy thing to forget, and therefore we should add a pre-commit check to find these early as possible.

The solution

Add a pre-commit check to look at HTML fragments and check that all nested element contain the required attribute.
See here for specification.

Definition of done

Check implemented with pre-commit that can catch forgotten markdown attributes.