Support exclusion of files from the config

Check file exculsion via Rubocop's `Config#file_to_exclude?` method.
Generally, I agree. Though, flipping the logic would make `relevant_file?` methods much harder to read. Example:
```ruby
file.ruby? &&
  !file.removed? &&
  !configuration.file_to_exclude?(file.filename)
```
I'm going with as-is for now, and watch to see how it plays out.