Add utilities for docformatter development

[Argmaster]

I'd love to see this project become something (very) big, therefore, I could, if welcome, provide in the form of PR a set of configurations for tools typically used by open source projects. By this I mean:

• tox
• flake8
• isort
• black
• mkdocs
• prettier
• pytest
• mypy
• pre-commit

Those ain't a random choice, I already configured them for multiple of my own projects, and I do like how they work together. I usually also include a set of requirements-*.txt files for requirement management. You can see what the effects more or less look like here.

I am open to further discussion on this topic.