Extend Doorkeeper TokenResponse processing

Optionally include OpenID Connect `id_token` in `TokenResponse`.

I don't much like making Doorkeeper aware of an OpenID Connect extension, but I don't see another way to inject an extra property into the `TokenResponse` body.  I like the use of `on_successful_authorization` in the `RequestConcern` to abstract the `TokenRequest` implementation, but the `TokenResponse` seems coupled to knowledge of `AccessToken` in the `TokenResponse`.

I plan to provide an implementation to `AccessToken#id_token` in the OpenID Connect extension gem I'm working on (https://github.com/playon/doorkeeper-openid_connect), but thought this might allow alternate implementations as needed.  I'm open to alternate extensions that allow me to inject additional members into the JSON response to a `TokenRequest`.  This was just the most straightforward way I could see to do it (I am not a Rubyist).