message: add thread_ts to outbound messages
diff: @@ -331,6 +331,7 @@ module.exports = function(botkit, config) {
  channel: message.channel,
  text: message.text || null,
  username: message.username || null,
  + thread_ts: message.thread_ts || null,
  parse: message.parse || null,
  link_names: message.link_names || null,
  attachments: message.attachments ?
  