Add ability to exclude files from the PII Scan

[JordoHeffernan]

As a user, I want to be able set the PII scan to ignore specific files in my repo

AC

1. Add flag --pii-exclude-file to scan command
   a. can be added multiple times
   b. flag takes file path as a string and passes it to pii-scan service to ignore
2. Add pii-exclude-file config to .secureli.yaml
   a. pii-scan respects this config
3. Update relevant documentation in README.md to reflect these changes

[blonienc]

Completed by @tristanl-slalom in this ticket: #558.