broadcast.go select sensor values to be sent to live chat

[Russell Stanley]

Currently, the latest water temperature signal can be sent to the youtube live chat. An enhancement would be to select any valid sensor when creating a broadcast and the respective signals are sent to the live chat. This will be particularly used when we can report visibility but can be used to serve any sensor reading to the live stream watchers.

[Russell Stanley]

Working on this

[Saxon Milton]

Merged in sensor-check (pull request #204)

Livechat sensor selection

Resolves issue #198

Approved-by: Saxon Milton Approved-by: Trek Hopton

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