Mirror private channels

[kpengboy]

Slackbridge should be able to mirror private channels too, as secret channels with restricted access.
Registration with NickServ would be a prerequisite for joining the corresponding IRC channel.

[jvperrin]

This is mentioned already in #7, but it's good to have a more specific issue for it.

[kkuehlz]

Yes, this is possible. When the bridge is invited to a private channel it could spin up an invite only IRC channel and add all corresponding slack bots. We could receive a member_joined_channel event to add newly joined users. And we only do this if the channel's is_private property is set.