Decouple checks from CLI

Right now, the code is totally coupled to stdout/stderr output and CLI usage.

Compare it to brakeman, for instance: Brakeman.run(app_path: Dir.pwd).filtered_warnings.

I'd like to be able of checking my code from my test suite. This is how I can do it with brakeman:

def test_brakeman
  tracker = Brakeman.run(app_path: Dir.pwd)
  warnings_count = tracker.filtered_warnings.size

  assert_equal 0, warnings_count
end

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