fix for download selected files only

[ghost]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[X] Bug fix
[ ] New feature
[ ] Other, please explain:

What changes did you make? (Give an overview)
This is a temporary fix for the open issue webtorrent/webtorrent#164 to make webtorrent-cli work as expected until the webtorrent team improves the API. It implements fix webtorrent/webtorrent#164 (comment), which is also referenced in the official webtorrent documentation as relevant (https://github.com/webtorrent/webtorrent/blob/master/docs/api.md#filedeselectpriority), so I decided to create a pull request with that fix.

Which issue (if any) does this pull request address?
argv.select has no effect and webtorrent-cli downloads the whole torrent instead, even while streaming. Users report this is an unwanted use of resources:

#222
#250

Is there anything you'd like reviewers to focus on?

[github-actions]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?