* Adds service delete command
This chageset adds `service delete` command, able to delete a service
at a time.
Fixes#49
```
kn service delete --help
Delete a service
Usage:
kn service delete <SERVICE_NAME> [flags]
Aliases:
delete, del, d
Examples:
# Delete a service 'svc1' in default namespace
kn service delete svc1
# Delete a service 'svc2' in 'ns1' namespace
kn service delete svc2 -n ns1
```
* Removes aliases, PreRunE and addresses review comments