Feature: Option to fail on unused/unneded configuration

[fenuks]

Before You Begin

Before proceeding, please make sure to follow these steps:

• I have checked for similar feature requests in the project's issue tracker
  to avoid duplicates.
• I have searched existing issues to see if this feature has been discussed
  before.

Feature Details

I would like to have a flag to fail if licensecheck configuration is outdated,
e.g. ignored package is no longer a dependency.

Is your feature request related to a problem? Please describe

There are few cases when licensecheck configuration can become obsolete:

• ignored package has been removed from dependencies:

[tool.licensecheck]
ignore_packages = [
    # package is no longer used, i.e. ignore is unnecessary
    'no-longer-used-package',
]

• ignored licence is now allowed by default:

[tool.licensecheck]
ignore_licenses = [
  # for now, licensecheck fails for ZPL-licensed packages, but it might
  # change in the future
  'Zope Public License',
]

• package without licence metadata has been updated:

[tool.licensecheck]
ignore_packages = [
    # current (5.11.0) ujson package is missing licence metadata,
    # but it has been fixed in the git version, so for the next release,
    # this ignore will be unnecessary
    'ujson',
]

Describe the solution you'd like

Please add a flag to return non-zero code if tool configuration is redundant,
e.g. --fail-on-redundant-configuration.