Implement support for 'select only' (so) magnet link parameter

[feross]

Originally proposed by @Daniel-Abrecht in webtorrent/webtorrent-desktop#83 (comment)

There's a BEP that we should probably support: http://www.bittorrent.org/beps/bep_0053.html

This magnet extension introduces the concept of an optional select-only file index array in the magnet URI, so that torrent clients can automatically select specific file(s) for download after the metadata has been downloaded.

This could probably replace the ix parameter that we invented for this exact use case. Since this is an actual BEP, it's probably better to support this instead (though it didn't exist when we invented ix)

[SilentBot1]

This has been implemented as of the commit 925bb88.

[Daniel-Abrecht]

Wow, this was really fast. Thanks everyone.

[feross]

Released as 0.100.0.