More control over exit code

In Azure Pipelines, the exit code of a command determines the success or failure of a task. If any task in the pipeline fails, the entire pipeline run shows as a failure. We would like our Bandit task (and therefore the build) to fail if high severity and high confidence warnings are found, however we want the task (and the build) to succeed if there are low or medium severity warnings.

We would like a command line option or entry in the config file to allow users to choose exactly how Bandit will exit. This will allow users to much more easily integrate Bandit into their CI pipelines without the need for extra scripts. Since by default all warnings will cause Bandit to exit 1, we would like a command line option to specify which levels to exit 0 on. For example, --exit-zero-severity l would cause low severity warnings to exit zero, and --exit-zero-confidence mh would cause medium and high confidence warnings to exit zero.

We've considered only reporting high severity warnings using -lll, however we still want to see the low and medium warnings in the artifact and this flag would not display them. We could also force Bandit to always return 0, but then we lose the visibility of having high severity items fail in the pipeline.

Start at Bandit's command-line option and configuration handling, then trace the code that determines the process exit status. Define how severity and confidence selections affect exit-zero behavior while preserving all findings in output; done means the options work for both CLI and configuration usage and are covered by the relevant existing tests.