* Bug fix: the pre-commit hook previously only scanned the working directory
rather than staged files. This release updates the pre-commit hook to instead
scan staged files so that git-secrets will detect violations if the working
directory drifts from the staging directory.
* Added the `--scan-history` subcommand so that you can scan your entire
git history for violations.
* Added the ability to filter false positives by using a .gitallowed file.
* Added support for `--cached`, `--no-index`, and `--untracked` to the `--scan`
subcommand.