Async validation

[alidavut]

Hello there. In some validations I need async operations but I think joiful doesn't implement 'external' and 'validateAsync' methods.

Is there any plan to implement? Or maybe I create a pull request if I can.

[laurence-myers]

I see that async validation was added in Joi v16.0.0 (1, 2). joiful v3.0.0 was released recently, which updated from Joi v15 to v17.

This is definitely something that joiful should support! If you have time, a PR would be great. 👍