rander should be injected with SetRand(r io.Reader) to be reproducible with seeds

[ugol]

I added xid to JR [1], which is a CLI random data generator.
As in google uuid [2], to be possible to use different rand algorithms rander should be refactored like this:

var rander      = rand.Reader

and

func SetRand(r io.Reader) {
	if r == nil {
		rander = rand.Reader
		return
	}
	rander = r
}

that could also be useful to generate sequences with a seed (to be reproducible), though time should be mocked too (if that it's interesting I can submit another patch)

[1] https://github.com/jrnd-io/jr
[2] https://github.com/google/uuid