Add RAM-only mode

[lhartmann]

Please add a mode that does not save anything to the backing hard disk.

Reasoning:

• Allows amnesiac usage, untraceable, no clues left on hard disk.
• Allows diskless usage.
• Allows streaming of huge files, larger than local storage.

I tried to use BTFS for clonezilla image transfers, but there is no disk available during that process, so it did not work.

[johang]

I believe you can achieve this by using the --data-directory parameter and set it to some tmpfs or ramfs.

[lhartmann]

Closing in favor of #59, which should solve both.