Add support for Markdown files with frontmatter

[mschoettle]

A stab at #382 with an ugly hack so far.

Also added tests for the behaviour fixed by #378.

• [ ] Added a change log entry in changelog.d/<directory>/.
• [x] Added self to copyright blurb of touched files.
• [x] Added self to AUTHORS.rst.
• [x] Wrote tests.
• [ ] Documented my changes in docs/man/ or elsewhere.
• [ ] My changes do not contradict the current specification.
• [x] I agree to license my contribution under the licenses indicated in the changed files.

[floriansnow]

Thank you for your contribution! What is the state of this PR? It's still marked as draft; does that mean you'll continue on it at some point?

[mschoettle]

@floriansnow I think I stopped because I was unsure if this solution is acceptable/the right one. Could you take a look and let me know if this makes sense? If so, I will finish it, otherwise take another stab.

[carmenbianca]

I've given this a little patch-over. It looks pretty good to me! Thanks for the contribution @mschoettle

[mschoettle]

@carmenbianca Thanks a lot for the help and finalizing it!