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();

[nrbnlulu]

@AdrianEddy I am maintaining a fork here if you want https://github.com/nrbnlulu/rffmpeg

[AdrianEddy]

@nrbnlulu I recently got rights to maintain this repository and I'm planning to merge this and some other outstanding PRs soon.
I need a few days to finish other things and I'll take care of this. If you have some specific needs, please open PRs here - we already have way too many ffmpeg forks - I'll do my best to merge all needed PRs to this repository