Adds option for recursively allowing additional props

[segphault]

This PR adds an option for recursively applying allowAdditionalProps, effectively making it possible to match on an object subset without having to splat any() on every single nested object.

Example:

assert.deepEquals(
  {foo: {bar: "baz", extra: true}, extra: true},
  {foo: {bar: "baz"}},
  {allowAdditionalProps: true})

I realize that this library hasn't been updated in awhile, so no worries if you aren't interested in accepting this. Feel free to close, I'm happy to just make a fork if that's easier.

[andywer]

Hey @segphault, thanks for contributing!

Yeah, I guess that makes sense 👍