Restic: Allow dumping files to stdout

[fd0]

This is a followup issue to #255: Find a place and implement a command or option that allows dumping a file from a snapshot to stdout.

This allows the following use case:

• Backup an SQL dump with restic: mysqldump | restic backup --stdin --stdin-filename production.sql
• Restore it directly: restic cat-file-to-stdout $SNAPSHOTID production.sql | mysql

[cfcs]

Also somewhat useful for workarounds like the one suggested in https://github.com/restic/restic/issues/949

[fd0]

This is done in #1346, there's the new dump command. I'm closing this issue.

[rawtaz]

Should have been restore --stdout instead of dump, to be coherent with backup --stdin.