This PR adds an option named `additional_file_patterns`.  You can specify custom path patterns (including globs) that the gem will use to annotate.

For example, I used it like this on a project:

```ruby
Annotate.set_defaults(
      'additional_file_patterns' => [
        File.join(Rails.application.root, 'app/lib/forms/%PLURALIZED_MODEL_NAME%/**/*.rb'),
        File.join(Rails.application.root, 'spec/lib/forms/%PLURALIZED_MODEL_NAME%/**/*.rb')
      ],
      ...
)
```

This makes it possible to have files nested under a directory which corresponds to the model.

I believe this fixes #594.