Add option to download Torrent on background

**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`](https://github.com/johnvictorfs/nyaa-cli/blob/master/nyaacli/torrenting.py#L14) function already has an argument to not show download progress (`show_progress`)