Wraps the ffpmeg cli, using -progress to report progress

Sometimes you just want a simple way to use ffmpeg. Most crates just use ffi, leading to complicated interfaces. ffmpeg_cli avoids this by wrapping the cli, for when you don't need the flexibility the real ffmpeg api gives you.

```rs use std::process::Stdio;

use ffmpeg_cli::{FfmpegBuilder, File, Parameter}; use futures::{future::ready, StreamExt};

[tokio::main]

async fn main() { let builder = FfmpegBuilder::new() .stderr(Stdio::piped()) .option(Parameter::Single("nostdin")) .option(Parameter::Single("y")) .input(File::new("input.mkv")) .output( File::new("output.mp4") .option(Parameter::KeyValue("vcodec", "libx265")) .option(Parameter::KeyValue("crf", "28")), );

let ffmpeg = builder.run().await.unwrap();

ffmpeg
    .progress
    .for_each(|x| {
        dbg!(x.unwrap());
        ready(())
    })
    .await;

let output = ffmpeg.process.wait_with_output().unwrap();

println!(
    "{}\nstderr:\n{}",
    output.status,
    std::str::from_utf8(&output.stderr).unwrap()
);

} ```