when() switch statement

```js
const schema = {
    a: Joi.number(),
    b: Joi.number().when('a', {
        switch: [
            { is: 1, then: Joi.number().min(1) },
            { is: 2, then: Joi.number.max(10) }
        ],
        otherwise: Joi.forbidden() });
```

Basically replace `is` and `then` with a new `switch`. Internally, it gets broken into multiple `when()` statements. This is just sugar.