Automatically leave channels

[lucymhdavies]

Related to, and follow-on from, #14

Iterate through channels that the welcomebot user is in, compare to channels in the config, and leave any channels welcomebot is not in.

https://api.slack.com/methods/channels.leave

Not sure if this is really needed, from a performance perspective. Depends how the bot monitors channels.

But worth doing, as a way of signalling to users in that channel that welcomebot is no longer listening.

Do not attempt to leave the equivalent of the "general" channel (or handle the cant_leave_general error)