Add Cookie.load_buffers

[ghost]

Adds a binding for magic_load_buffers. Requires rust-magic-sys#10.

Intended to be used like this:

let cookie = magic::Cookie::open(magic::CookieFlags::default());
let magic_data = std::fs::read("/usr/share/misc/magic.mgc").unwrap();
let buffers = vec![magic_data.as_slice()];
cookie.load_buffers(&*buffers).unwrap();
println!("{:?}", cookie.file("/usr/share/misc/magic.mgc"));