Implement custom stream IO

[AdrianEddy]

This PR adds a way to create inputs and outputs by providing a standard Rust Read/Write/Seek streams.

It allows using ffmpeg with arbitrary data sources like filesystem, network, memory etc

examples:

let file = std::fs::File::open("test.mp4")?;
let input_ctx = format::input_from_stream(format::context::StreamIo::from_read_seek(file)?, None, None)?;

let mut vec = Vec::new();
let mut cursor = std::io::Cursor::new(&mut vec);
let mut output_ctx = format::output_to_stream(format::context::StreamIo::from_read_write_seek(cursor)?, None, Some("mp4")).unwrap();