[ENH] Add --diff option to show diff when running with --check

[tdgroot]

We use mdformat to validate Markdown formatting in a documentation project we're running. Most users are contributing to the documentation by creating changes, commits and pull requests from the Github UI, so they won't be running any linting/validation on commit. In this case, it would be very useful for those users to see what mdformat is marking as "unformatted" and knowing what should be done to fix that.

This was highly inspired by the implementation that Black uses, as it seems to doing just what we need.

Looking forward to your thoughts and review :).

[welcome]

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[AlexanderGrooff]

Would be great to have this feature! 🚀

[tdgroot]

@hukkin any chance to get a review for this PR? :)