[audit functionality] Return the correct occurrence of the secret text in a line

[KevinHock]

If you have e.g.

self.thepassword = "thepassword" 

and run the soon-to-be-merged keyword detector, then run the audit functionality, it will highlight the first occurrence, and instead of the second. This is a bug.

This is because in audit.py we find the index of the secret

detect-secrets/detect_secrets/core/audit.py

Line 561 in 1415b4b

index_of_secret = secret_line.lower().index(raw_secret.lower())

and we don't return the index of the secret in the secret_generator method of plugins.

This somewhat related to the issue of handling multiple secrets on the same line

detect-secrets/detect_secrets/plugins/base.py

Line 76 in 1415b4b

# TODO: Handle multiple secrets on single line.

[KevinHock]

On a related note, w/r/t the Keyword detector, the biggest source of not specific-to-yelp false-positives are variables like FORGOT_PASSWORD: 'forgot_password' where LHS == RHS, maybe I shouldn't flag these, as I've only seen false-positives, but not catching password = 'password' would irk me.

[lorenzodb1]

Closing as duplicate of #493.