add a force exclude option

Related to #33. This option is solely to mirror black's CLI.

Edit: turns out this is for files that were explicitly listed in the command line:

python -m blackdoc --check --force-exclude "to_format.py" . to_format.py

would not reformat to_format.py while --exclude would only filter the to_format.py from reformatting .

[ ] Tests added
[x] Passes isort . && black . && flake8
[x] User visible changes (including notable bug fixes) are documented in changelog.rst
[ ] New features are documented in the docs

keewis / blackdoc