update `specific-test` flag on scan command to include custom hooks

[JordoHeffernan]

As a user, I want to be able to only run one custom scan, similar to running only one pre-commit hook

AC

1. scan command flag --specific-test and -t accepts both pre-commit hook ids and ids for our custom scans
2. custom scan ids are check-pii and check-regex
3. If one of these are passed in to the flag, it will run that scan, otherwise move on to current functionality of running the indicated pre-commit-hook.
4. Update documentation in README and relevant functions to clarify this functionality

Notes

Dependent on completion of #460 and #435