Added withJsValidator

[bytestream]

Mimics Laravel's withValidator function, see https://laravel.com/docs/5.8/validation

Adding After Hooks To 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!');
        }
    });
}