automatic FULL backup

[gsmolk]

If backup process is streamlined via automation system such as puppet or ansible, then adding new instance to backup schedule in the middle of the week can be painful, because only incremental backups are scheduled during this time.
So it would be good to have an opportunity to automatically create FULL backup if there is none, even if --backup-mode is explicitly set to DELTA or PAGE. This behavior should be enabled via flag --automatic

[gsmolk]

Additionally, it would be good to automatically choose the incremental mode.

[gsmolk]

Will be done as part of #143