I was looking at #859 and thought that it seemed like something that belonged in the core of Joi, not as a plugin/extension and had a pretty good idea of how I would implement it. This PR contains that initial implementation.

What It Does

It allows a user to specify a comparator function which is used only for objects.

// Compare objects with custom logic
const objectComparator = (left, right) => {

  return left.username.toLowerCase() = right.username.toLowerCase();
};

const objectSchema = Joi.array().unique({
  objectComparator: objectComparator
});

Joi.attempt([{ username: 'bob' }, { username: 'BoB' }], objectSchema); // throws error
const result = Joi.attempt([{ username: 'bob', name: 'Bob' }, { username: 'bob-2', name: 'Bob' }], objectSchema); // result -> [{ username: 'bob' }, { username: 'bob-2' }]

In the event that a comparator isn't provided it just defaults to the standard behavior using Hoek.deepEqual() so the change has a default and also is backwards compatible.

What It Doesn't Do

It doesn't allow a comparator to be provided for types other than object. Originally I wanted to make the parameter just comparator and use it for everything, but once I started implementing this I realized I couldn't do that without a noticeable decrease in performance because of how it works right now using an object map.

The only cases I can think of where a comparator function would be useful for things other than objects is when the data needs to be modified, which that could just be done prior to validation. For example, if an array of numbers needs to have Math.floor() run on each value and in that case I imagine you would want the value to come out of validation already run through Math.floor() anyways. Others would be string manipulation and such like lowercase comparisons, etc.