`backup`: Add option which command to call for stdin backup

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 ..."
```