Warn that a huddle as started

[Konubinix]

When a huddle is started, there is no event in matrix and the logs of the bridge during the time of the huddle are

DBG Starting handling of XXX by USLACKBOT, subtype huddle_thread module=Portal/XXX
WRN Received unknown message subtype huddle_thread module=Portal/XXX
WRN unknown message {unmarshalling_error RTM Error: Received unmapped event "huddle_invite"} module=User/XXX
WRN unknown message {unmarshalling_error RTM Error: Received unmapped event "user_huddle_changed"} module=User/XXX
WRN unknown message {unmarshalling_error RTM Error: Received unmapped event "huddle_invite_cancel"} module=User/XXX
WRN unknown message {unmarshalling_error RTM Error: Received unmapped event "user_huddle_changed"} module=User/XXX
WRN unknown message {unmarshalling_error RTM Error: Received unmapped event "user_huddle_changed"} module=User/XXX
WRN unknown message {unmarshalling_error RTM Error: Received unmapped event "user_huddle_changed"} module=User/XXX
WRN unknown message {unmarshalling_error RTM Error: Received unmapped event "user_huddle_changed"} module=User/XXX
WRN unknown message {unmarshalling_error RTM Error: Received unmapped event "user_huddle_changed"} module=User/XXX

I would have expected to at least see a message that would suggest me to open my slack app not to miss the huddle.

[Konubinix]

By looking at the code, I understand that this needs also an improvement at https://github.com/beeper/slackgo so that it maps huddle event in https://github.com/beeper/slackgo/blob/36e35976c9c3d6d5c833713c860b762d3f258eb8/websocket_managed_conn.go#L532