utilities/tp-link-to-influxdb#4: Support for a persistent container

Although I'd have preferred it to be a commandline flag (so it's obvious why a cron job isn't exiting) that's likely to be less compatible with things like Synology.

So, I think this needs to be an option in the config file.

Currently, example config looks like this

# List tapo devices
tapo:
    # Tapo devices require that you log in with the credentials
    # that you use to log into the app
    #
    user: "me@mymail.com"
    passw: "mysecretpass"
    devices:
        - 
            name: "washing-machine"
            ip : 192.168.3.152

        - 
            name: "big-fridge"
            ip : 192.168.3.153

# List kasa devices
kasa:
    devices:
        - 
            name: "desk-plug"
            ip: 192.168.3.150

        - 
            name: "tumble-dryer"
            ip : 192.168.3.151


# InfluxDB outputs
influxdb:
    -
        name: "Cloud"
        url: "https://foo.example.com"
        token: "aaabbbccc=="
        org: "my org"
        bucket: "telegraf"

I think the best way to go at this is add a dedicated poller section to contain config for the poller itself, with backwards compatible defaults used so that any existing configs still work after the change

Have added support for a persistent run-mode.

The necessary config is

poller:
    # If true, run in an infinite loop
    persist: true
    # Interval in seconds between runs
    interval: 20