Add skip_doorkeeper_for (and other refactors)

- use `ActiveSupport::Concern` refactor `Doorkeeper::Helpers::Filter`
- add option `id` to doorkeeper_for to name the bearer
- add `skip_doorkeeper_for`

this change can give an advantage:

```ruby
class BaseController
  doorkeeper_for :all, id: :global
end

class Sub1Controller < BaseController
  skip_doorkeeper_for :global, only: :index

  def index; end;
  def show; end;
end

class Sub2Controller < BaseController
  def index; end;
  def show; end;
end
```
Something like this is what I meant indeed! But actually, if `@name` is always the same value (and there's little reason to make it configurable), why not just use the string in place, and removing the new attribute? Let's simplify it.