Add audio feature for crate

[attila-lin]

Hello~

In my use case, I don't need to play the audio.

So add a audio feature to play the audio which contains the deps for this job will be nice for users like me to reduce the deps.

[kxxt]

Actually I wanted to separate the dependencies for the bin and lib from the very start of RIIR. Unfortunately it is not possible(rust-lang/cargo#1982).

So add a audio feature to play the audio which contains the deps for this job will be nice for users like me to reduce the deps.

It is the best workaround we can use for now. I will add a binary feature so that clap, env_logger and rodio are not required for the lib.

I don't need to play the audio.

Actually the rust lib doesn't have any API to play the audio. I have no plan to support it until there is an asynchronous playback API. rodio uses a multi-thread approach and it won't play well with tokio.

[kxxt]

Implemented in v5.1.0.