Add skip_doorkeeper_for (and other refactors)

- Use `ActiveSupport::Concern` to refactor `Doorkeeper::Helpers::Filter`.
- Add `skip_doorkeeper_for`.

## Roadmap

1. [ ] Polish PR until it is ready, keeping the `doorkeeper_for` API.
2. [ ] Add a deprecation notice when on the `doorkeeper_for` call, saying that this way of introducing doorkeeper is deprecated and showing the new usage "(Rails example...)", etc. See https://github.com/doorkeeper-gem/doorkeeper/pull/434#issuecomment-49895704.
3. [ ] Merge and release a minor version with your enhancement.
4. [ ] See if there's other backward incompatible changes in the pipeline, and release a major version with the removal of `doorkeeper_for` in favor of `before_action :doorkeeper_authorize!`, and the other deprecation `test_redirect_uri`.
Let's pick _one_ standard. :)