modify file paths via magnet link parameter

[milahu]

BEP53 (#63) allows to download only some files of a torrent

in some cases, it would be useful to also modify the file paths
for example from some_folder/some_ugly_name to some.pretty.name.txt

this allows to split multi-file torrents into multiple single-file torrents

possible solution
mvi=1:new/path/1.txt - move from file index
mv=old/path/1.txt:new/path/1.txt - move from file path. related: #90 #92

example use case: download individual files from libgen torrents
the ugly filenames are md5 hashes (content-addressed storage)
the pretty filenames are based on separate metadata

-0/003ae617fbaba357b52f2f0c2b6a21dd
+Gyorgy Buzsaki - Rhythms of the brain (2006, Oxford University Press, USA).pdf