Add callback to signal $INCLUDEs

[k0ekk0ek]

NLnetLabs/nsd#57 describes how NSD fails to detect zone files changed if an $INCLUDEed file is updated. The parser concerns itself with deserialization and does not maintain any contextual state. The best approach seems to add an additional callback that is called whenever an included file is closed so that the application can register any dependencies that may exist. Registering the callback should not be required for normal operation.