Add "display" param to OAuth2/Facebook auth provider

Facebook supports a `display` param - see here https://developers.facebook.com/docs/reference/dialogs/oauth. Very useful when showing the authentication page in a popup.

Btw: The [securesocial](https://github.com/jaliss/securesocial/) plugin introduced an `authorizationUrlParams` and an `accessTokenUrlParams` setting where any value can be set (see [the documentation](https://github.com/jaliss/securesocial/blob/master/docs/src/manual/source/guide/configuration.md#oauth-2-providers-extra-configuration)). Maybe this would be a good solution in the future for the play-authenticate module as well?

@joscha What do you think?