More convenient output structure for 'sd seedvault'

[MrHernicle]

It fits my use case to have all generated torrent files, the cover image, and the log files for each discs in one directory, preferably wih roughly the same nameing scheme.

[thijzert]

To quote the Spice Girls: So, tell me what you want, what you really really want, either as prose or as a PR.