Update how responses are flushed for Express 4.x.
Support config option `customizeResponse` to allow modifying grant response.

By using a truthy value for this config option, the access token grant
middleware will *not* send a response. Instead, it will attach the grant
object itself (`grant`) and the intended response (`oauthResponse`) to
the request object and pass onto the next middleware. You'll want to
ensure that you provide additional middleware after the `grant()` in
your Express route.