Add configuration option for apns-push-type

[konrad-xtention]

For some usecases it would be helpful if it was possible to configure the apns-push-type that is sent to Firebase (and subsequently to APNS) via the Hedwig config, the Pusher, or both. Sensible configuration values would be alert, background and none (default, same behavior as now).

Note: The Readme says Hedwig sets "apns-push-type":"background" which seems to be not true.