Make default opt-in be group-wise.

[tarcisioe]

Each group should be able to decide if the bot default is opt-in or opt-out in that group.

[JPTIZ]

Not sure if this is really wanted from the perspective of someone joining a group and unadvertely be tracked without consent. But considering Karmagram doesn't store sensible information (just countings, AFAIK), shouldn't be much problem, but still...

[cauebs]

Maybe if we add a command to purge all data related to you this won't be a real concern.