feat: allow to have config at pyproject.toml

[tinovyatkin]

This PR allows to specify options at pyproject.toml file (at tool.mdformat section) as most modern Python tooling (black, ruff, etc) doing.

Fixes #356

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[tinovyatkin]

@hukkin could you take a look please?

[dynobo]

FYI: While I'd love to see this integrated in mdformat as well, in the meanwhile I learned through #379 about the plugin mdformat-pyproject, which serves me well in the meanwhile.

[sanmai-NL]

@hukkin ping