Allow vessel ID to be included in topic

[tkeffer]

continuing the discussion from Issue #46...

While the plugin allows the user to specify which SignalK paths are to be published, they are all published to topic signalk/delta. This pushes the responsibility for disambiguating vessel sources to the subscribing clients. They have to listen to everything, even vessels that they are not interested in.

Suggestion: publish to signalk/{vessel}/delta where {vessel} is the MMSI or UID of the SignalK server. Or, even better, publish to path-specific topics, such as signalk/{vessel}/delta/navigation.position. Then the client can target exactly what it needs.

@tkurki Suggests

One way to achieve by path and optional by source subscription would be NOT to map value paths to hierarchical MQTT topics but simply two levels: per the previous example: vessels/<id>/navigation.position/n2k-sample-data.160. This would allow subsciptions to

1. all values for a path for a vessel, from any source with vessels/<id>/navigation.position/+
2. all values for a path for any vessel, from any source with vessels/+/navigation.position/+
3. all values from a source on a vessel: vessels/<id>/+/n2k-sample-data.160

but not allow hierarchical subscriptions, like all navigation values for vessel x.
We could also do a hybrid: first 2 value path levels as hierarchy, like vessels/<id>/batteries/1/capacity.remaining/sourceId. This would allow subscribing to all values for a device where the id is in the path, like a battery. Not very intuitive though.

Option 1 is the important one. Option 2 could happen when you're pretty confident only one vessel will be on the broker and don't want to be bothered looking up the <id>. Option 3 seems unlikely, or at least not a priority.

Sources are good at different things. A GPS sensor like the ZG100 will give you 2 m accuracy, but it sucks as a compass. So, it would be less of a priority for me to say, "Give me everything from the ZG100."

The notation navigation.position[n2k-sample-data.160] is the one suggested in the SK docs. I think it's clunky, but it does exist.