Add hook for validator before resolution

[hailwood]

The same as you can use the withValidator method on form requests, we can now do that with mutations.

From the Laravel docs on this method on form requests:

If you would like to add an "after" hook to a form request, you may use the withValidator method. This method receives the fully constructed validator, allowing you to call any of its methods before the validation rules are actually evaluated:

/**
 * Configure the validator instance.
 *
 * @param  \Illuminate\Validation\Validator  $validator
 * @return void
 */
public function withValidator($validator)
{
    $validator->after(function ($validator) {
        if ($this->somethingElseIsInvalid()) {
            $validator->errors()->add('field', 'Something is wrong with this field!');
        }
    });
}

Additionally we're also passing through the $args as the second parameter.