Slack thread support

[tovine]

As it is now the replies to threads in slack just appear as normal messages in IRC, this can be a bit confusing.
I would suggest to mark the messages with something like (Reply to thread: "Thread name [maybe cut off with (...) if over a certain length]") as a good starting point solution.
Now I don't have any experience with the Slack API (nor much experience with with Node) so I don't know if it's easily done, but the task itself seems relatively simple.

As a long-term goal the bot could also allow responding to a certain thread from IRC? I'm not sure how that one should be done though, but it could be something to explore - maybe generating a specific "id" that you would prefix the IRC message with when typing a response?