Allow threading for the ack and result messages for the slack backend

[skiedude]

Fixes: https://github.com/StackStorm/hubot-stackstorm/issues/161

Example action alias definition

ack:
  extra:
    slack:
      thread_response: true
  format: ":speech_balloon: ACK, Looking that up for you..."
  append_url: false
result:
  extra:
    slack:
      thread_response: true
  format: |
    On-Call:
    Name: {{execution.result.output.result.name}}
    Username: `@{{execution.result.output.result.user_name}}`
    Phone: {{execution.result.output.result.mobile_phone}}

Hubot-slack checks for the existence of envelope.message.thread_ts in order to thread the response instead of sending to the open channel https://github.com/slackapi/hubot-slack/blob/main/src/client.coffee#L191