Disable pet per chat

[ForNeVeR]

Not everyone wants a pet plugin to be active in their MUC. We should add a configuraion option to disable pet (or to enable it) in the selected chats.

[ttldtor]

Maybe we should make all plugins disableable?

[ForNeVeR]

Yes that's an ongoing idea, see #170. But disabling the pet plugin is somewhat more urgent than a more general mechanism.