Add markdown linters

[jkemp101]

Should we add markdown linters like the following. Ideally there would be a python one we could use.
https://github.com/executablebooks/mdformat

https://github.com/DavidAnson/markdownlint
https://github.com/markdownlint/markdownlint
https://github.com/igorshubovych/markdownlint-cli

[tinvaan]

@jkemp101 markdown support has landed in #133, feel free to close this issue now.

[vtclose]

Unfortunately I had to revert part of #133 in #143 due to issues in mdformat. I'll re-add it later when the issues will be resolved.

[tinvaan]

@vtclose oh, do you mind creating an issue for this? If I can get an example and your desired use case, I'm happy to attempt a patch to upstream.

[vtclose]

There's an existing discussion in executablebooks/mdformat#359. It seems like might be unwilling to add support for ignores.