backup: Add option which command to call for stdin backup

[aawsome]

There could be an option like stdin-command such that you could run:
rustic backup - --stdin-command "echo test"
(which, of course, is equivalent to running echo test | rustic backup -).

The advantage is that you can also use this in your config file, e.g.

# makes a DB snapshot
[backup.sources]
source = "-"
stdin-command = "mysqldump ..."