Creates a OneSignal push notification

Parameter

• member => single member or list of members
• contents => %{key: value} => see OneSignal api docs for contents push notification map key. This has been kept for retrocompatibility, passing contents as a custom_data map key should now be preferred
• custom_data => %{key: value} => see OneSignal api docs for push notifications. These are options for the soon-to-be-created notification