read settings from a config file

[keewis]

This changes the CLI to read default configuration settings from a user provided config file or the project's pyproject.toml (if it exists).

For that to work, we have to run the argparse parser twice. Should we switch to click, which is used by black?

• Closes blackdoc reads and uses black setting in pyproject.toml. #40
• Tests added
• Passes isort . && black . && flake8
• User visible changes (including notable bug fixes) are documented in changelog.rst
• New features are documented in the docs