Create command for users to submit custom regex pattern for security scan

[JordoHeffernan]

As a user, I want to be able to have SeCureLi scan for custom patterns specific for my security concerns

AC

1. Add a new item to .secureli.yaml called custom_scan_patterns that is an empty list
2. ~~Create a new command called add_custom_scan_pattern (name suggestions?)~~ Expand Update command to include pattern additions
3. Command accepts a flag --new_pattern which can be added multiple times, each flag accepts a string
4. User input is checked and must be a regex pattern. if it pass the check, it is saved in config as an item in custom_scan_patterns a. If it fails, present error message to user
5. When the pre-commit scan runs, in addition to default security checks, and the items check by work completed in #435 also check for all patterns in custom_scan_patterns
6. Add documentation - Readme

Notes

• Rather than creating this as a new command, we could make it a prompt as a part of init and/or update