Add option for tracker subdirectory

Sometimes you have a pile of torrents, that you set up in a watch directory on a seedbox a while ago, and forgot about, and need to move them around, but rtorrent has already renamed the files to their hashed forms. Or maybe that's just me. To further classify my torrents, I added an option to further separate unhashified torrents by tracker name. Useful for when there are a bunch of torrents needing renaming and you aren't sure from where you got them. Maybe it'll also be useful to you?