Based on discussions for issue #10, add back the registry with processing rules, but make it's use dependent on a `registry` option, either loading the default registry, or a custom registry loaded from a URL. If not provided, the registry defaults to an empty object `{}`. All processing rules remain, but will not be invoked without a non-empty registry.

This does not simplify processing, but allows a behavior in which no such processing occurs.

Additionally, change the vocab_expansion option to `vocab_entailment`, and make sure that it is only invoked if that option is provided to the processor.