Subcommand aliases

[sviridov-alexey]

Allow define aliases for subcommands.
Rationale

• Sometimes different names of some subcommand better match different use cases. I.e. backup restore-snapshot and backup rollback-to have some meaning but one of them can be more expressive in different cases
• For deprecation reasons. For example, there was the command backup which is backing up the directory. After some time one decide to add the command backup-file and rename the first one to backup-dir, but still leave backup as alias of backup-dir for compatibility