Threading support for slack webook

[Mayank Malik]

This is a feature request.
A way to reply in a slack thread by providing a timestamp, a channel ID, and the Slack API Key in the command line.

[Sandeep Singh]

This can be optionally supported, for example when the user specifies slack_threads: true under config block.

slack: true
slack_webhook_url: https://a.b.c
slack_username: test
slack_channel: test
slack_threads: true

Reference:- https://api.slack.com/messaging/webhooks#threads