--force option does not exist for set offsets

[swenzel]

When trying to change the offsets of a non-empty consumergroup, the user will be prompted with a message saying that they should use the --force option to forcefully set the consumergroup offsets. That option does not exist and from what I can see is also not necessary to overwrite the offsets. Just answering "yes" to "Are you sure?" is enough.
We should make sure that the option does exist and is needed to do that. We should also inform the user that the effect this has on the consumers is undefined and depends on their implementation. I expect in most cases this won't have any effect at all since the consumers won't get notified of any offset change and will just continue consuming where they currently are. They will then go on committing their current offset again, thereby overwriting the manual overwrite again.