Add --max-warnings [int] option

[florblue]

Description

It was added --max-warnings [int] option: Number of warnings to trigger nonzero exit code.

It resolves issue #56.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update
• Existing unit tests pass locally with my changes

[coveralls]

Coverage remained the same at 99.657% when pulling 6fdefd6 on Cambalab:add-contribution-max-warnings-issue#56 into 2263ccb on protofire:master.

[fvictorio]

Please resolve the merge conflicts.

[fvictorio]

After the merge, using -q breaks the --max-warnings option. I understand -q as hiding the warnings in the report, not removing them altogether, so I think the two options should work together.