Add channel support to Pushbullet Alert Transport

Hi all

Struck me (as I was receiving 3000 alerts due to a slightly oversensitive ruleset) that it would be great to be able to receive Pushbullet notifications in a channel - I see the API is capable, I can even see what needs to be passed to the API to direct it to a channel…

curl -u <api_token>: -X POST --header ‘Content-Type: application/json’ --data-binary ‘{“type”: “note”, “title”: “Note Title”, “body”: “Note Body”,“channel_tag”: “<channel_id>”}’

But my woeful lack of PHP skills lets me down in working out how to make that happen in /includes/alerts/transport.pushbullet.php

I’m going to experiment - obviously if I succeed, I’ll submit the code.

edited to add if the channel_id is blank above - it behaves as if there is no channel_id - so as it is now.