Implement find file priority feature if `calibredb-preferred-format` is non-nil

[chenyanming]

What do you think group all formats into one entry, just like how calibre gui does, to treat multiple format books.

I think that would be a good idea actually and have the default format for calibre-find-file be "epub" (if nov.el is installed) > "pdf" > "others".

Originally posted by @tpeacock19 in https://github.com/chenyanming/calibredb.el/issues/24#issuecomment-839354922