Support BEP53 magnet link extension

[PastaPastaPasta]

BEP 53 support has been in webtorrent/webtorrent since webtorrent/webtorrent#1396

BEP 53 instructs a client to ONLY download certain file indexes, so specifying so=0 will result in only downloading the first file, and so=0-9 will result in the first 10 files being downloaded.

However, based on recent testing of mine, it does not appear that webtorrent-desktop supports this extension. Is there any possibility to enable this functionality in webtorrent-desktop?