Control report severity level separately to exit code

[robertlagrant]

Is your feature request related to a problem? Please describe.
Currently in bandit the minimum severity for reporting and for triggering an exit code of 1 is always the same - controlled by -l, -ll and -lll. This means, for example, that I cannot run bandit in CI with all vulnerabilities going into a report, while failing the CI job on more severe issues.

Describe the solution you'd like
Being able to control the two separately. For example, -lll can still control both, but perhaps having presence of equivalent -L flags overriding what triggers an exit code of 1. So here's my example above in this format:

bandit -r code_dir -l -LLL

Describe alternatives you've considered
I could just run bandit twice: the first time reporting everything with --exit-zero; the second time just reporting high severity and allowing the exit code to influence CI.

Additional context
This is part of how I'd like our CI to work whereby I use the excellent Microsoft open source Bandit SARIF formatter to create a SARIF report, which pushes to Github security issues.

[maciejstromich]

dup of #640?