Allow flexible mapping between files in a torrent and files on the file system

Allow specifying per-file where to download to/seed from. For a single torrent file with the files a/a.ext and a/b.ext, it should be possible to back them with /whatever/cool.ext and /somewhere_else/not_so_cool.ext. This mapping should be changeable at runtime, especially after a torrent has finished downloading and has been moved.