Add CLI option to disable default vendor

As I'm still unable to install debs generated with '+fpm0' stensa I've added a '--no-default-vendor' CLI option to disable the use of fpm as vendor if none is provided. This uses code reverted in d529051e0d88fab0c4ddbcd36b53ced916561781 in case the option is used and otherwise behaves as current master does.

I'm happy to add more commits to implement any suggestion or fix any regression (there seems to be an obvious one with regards to default revision too). Looking forward to discuss this further (if needed).