Add option to download Torrent on background

[johnvictorfs]

Edit: Meanwhile you can use the --c (--client) flag to download the torrent using your preferred torrent client

There should probably be some way to check ongoing downloads if such an option is added

Possibly something like nyaa --ongoing or nyaa --status

The nyaacli.torrenting.download_torrent function already has an argument to not show download progress (show_progress)

[johnvictorfs]

Relates to https://github.com/johnvictorfs/nyaa-cli/pull/31