Support jjv options like "checkRequired" and "removeAdditional"

[mike-marcacci]

Hey there! I have to say I'm quite impressed by the rollback strategy used here when applying defaults. I'd like to make a quick feature request that would make themis an easy candidate to replace JJV, which I currently use in most projects requiring json-schema validation:

JJV's validation options checkRequired and removeAdditional are extremely convenient, and probably aren't too hard to implement. I'll look into making a PR if I can find the time, but wanted to suggest these here anyhow.