Enable support for extended output

[daidoji]

Maybe this is already possible but this library should enable the extended output functionality of the base crate if it does not. If it does, if shown how I can update the docs.

You can do this in the base crate for example

// Extended output. OutputReader also implements Read and Seek.
let mut output = [0; 1000];
let mut output_reader = hasher.finalize_xof();
output_reader.fill(&mut output);
assert_eq!(hash1, output[..32]);