Add support for building with pre-seeded rngs

[alec-deason]

I'd love to be able to build an instance of BlueNoise using a presseded rng. I specifically want to avoid invoking get_entropy because that requires me to do more work in a minimalist WASM project I'm working on in order for getrandom to function.

This changeset is the least invasive way I could think to expose the functionality I want.

A more invasive but possibly cleaner solution, which I'm happy to write up if you prefer it, would be to create a true builder for BlueNoise which would allow for setting the rng before it needs to be initialized.

[arlyon]

A first draft of this was using a builder I'd say we're still right on the edge before it would be worth it. The changes seem reasonable :) I'll merge and release, I need to update the readme on crates.io anyways.

Thanks!