Allow `unpublish` command to skip user prompt if given `-force` flag

[isc-dchui]

Some of the integration tests publish to a registry and it would be nice to be able to clean this up by unpublishing. Unfortunately unpublish currently requires a user to respond. This seems like the perfect case for adding a -force flag that skips this step. It should still respect the repo settings about whether unpublish is allowed.

Also there should be better documentation that unpublish accepts a repository name using the unpublish <repository>/<module-name> <module-version> syntax