Add support for sending thread_broadcast

[dstrates]

Description

Would like the ability to send a threaded reply notification to a channel with thread_broadcast.

This could be surfaced as an optional flag to broadcast the threaded message to the channel, as mentioned in #119

What type of issue is this? (place an x in one of the [ ])

• [ ] bug
• [x] enhancement (feature request)
• [ ] question
• [ ] documentation related
• [ ] example code related
• [ ] testing related
• [ ] discussion

Requirements (place an x in each of the [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

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