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
```
I see. I've been playing in your branch, and the trivial implementation I had in mind:
```ruby
        def skip_doorkeeper_for(*args)
          skip_before_filter :doorkeeper_for_before_filter, *args
        end
```
Would work if we required and `:only` option key for cases like `skip_doorkeeper_for :index` (transforming it into `skip_doorkeeper_for only: :index`).