Pushbullet Channel Support

Let me start by saying, I just found ntfy... its AWESOME. Thanks for this great modular solution!

Now, onto the issue/feature request.

Within pushbullet, each account can create "Channels" to which other users can subscribe. Only the channel's owner can push to a channel. It would be nice to be able to specify pushing to a channel instead of to the user's feed as it works now.

I imagine this working through an optional parameter. If the optional parameter is not present, then the pushbullet backend works just as it does now by pushing to the users feed. With the optional parameter, the end user specifies the channel name (or possibly a YAML list to pass to multiple channels at once). The message is then pushed to all specified channels in the optional parameter.

If this functionality is present, I don't see it in the docs. If it isn't present, I think it would make a great (and probably fairly easy) feature request!