Feature Request: Add support for threaded conversations

[PaulAngus]

It would be great if a user (me) could send threaded messages

In its simplest form, i believe that it would simply be a case of allowing the user (me) to pass a 'thread_ts' value when posting a message, which would make the message a child of the first message in the thread (assuming that I passed the correct ts) and the thread_broadcast boolean, which (for my use case I would set to true).

I have a slack channel that receives updates from out CI system (using slack-cli). Builds run a series of tests, but as up to 30 builds can be running at once, it would be awesome if I could thread them...