Add consumer groups
Add consumer groups

The committed offset system needs to support consumer groups. This also
simplifies the code to use a helper function.

husio/kafka,taheris/kafka,legionus/kafka,danieltahara/kafka,alexcesaro/kafka,taheris/kafka,husio/kafka,alexcesaro/kafka,dropbox/kafka,zorkian/kafka,zorkian/kafka,optiopay/kafka,legionus/kafka,liftoffio/go-kafka,optiopay/kafka,dropbox/kafka,liftoffio/go-kafka,danieltahara/kafka