Save subscriptions on change

[david-i-berry]

Topics added to the active subscriptions need to be saved so that on restart the user does not need to re-subscribe manually.

[maaikelimper]

create topics.json with topics separate from config.json

[david-i-berry]

Example json file:

{
    "topics": {
        "origin/a/wis2/+/data/core/weather/surface-based-observations/synop": "$TOPIC"
    }, 
    "client_id": "20363bd5-d436-40ee-a60f-b135076b6bec"
}

I've included a client / session id for persisting between runs. Path to file specified in config.json (defaulting to .mqtt_session.json if not specified).

config.json:

{
    ...
    "session_info": ".mqtt_session.json"
    ...
}

[maaikelimper]

subscriptions are save in a mqtt_session_info.json

#18