add WavSpec chaining methods

[ozpv]

Adds some methods that makes code easier to write

changing a sample rate before e.g.:

let r = WavReader::open("signal.wav").unwrap();
let mut w = WavWriter::create("output.wav", {
    let mut spec = r.spec();
    spec.sample_rate = 8_000;
    spec
})
.unwrap();

After

let r = WavReader::open("signal.wav").unwrap();
let mut w = WavWriter::create("output.wav", r.spec().sample_rate(8_000)).unwrap();