Make `--gitignore` or `skip_gitignore` default behaviour (for black and all other profiles)

Black ignores files not version-controlled https://github.com/psf/black/blob/master/src/black/__init__.py#L613, as such profile black should imply https://pycqa.github.io/isort/docs/configuration/options/#skip-gitignore being true here https://github.com/PyCQA/isort/blob/1e0eb0d2aba1e5384724bac410efde44cbd57fbf/isort/profiles.py#L4-L11.