add seed support

[khrome]

Adding support for setting a seed in the RNG so you can have deterministic output.

This should probably be followed up by tighter testing using seeds to validate the final form of the output.

I'd also like to update this lib to use ESM natively and compile to commonjs in the future, but that seemed out of scope for this change.

Thanks!

[khrome]

You can see the more sgnificant update here, which I'd be happy to discuss, but is a major change to the project and not something I'd just throw a PR against. Thanks.