To extract audio

[imcrown]

Hey buddy can you add feature for only extract audio because while downloading and convert it take much time and network if we get only audio download feature so it will take less time to encoding and less network pls

[bytedream]

Duplicate of #424 (TLDR; i'm still considering to add it but nothing definite yet).

See #340 (comment) for a workaround with the search subcommand.