add channel_follower_count

[topi-banana]

https://github.com/GyrosOfWar/youtube-dl-rs/issues/38

let video = YoutubeDl::new(channel_url)
    .socket_timeout("15")
    .run()
    .unwrap()
    .into_single_video()
    .unwrap();

println!("subscribers count: {}", video.channel_follower_count.unwrap());