Support intersections

#### Support plan

* *is this issue currently blocking your project?* yes
* *is this issue affecting a production system?* no

#### Context

* *node version*: v12.4.0
* *module version*: joi 17.2.1
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): standalone
* *any other relevant information*: n/a

#### How can we help?

Hi!

I can't find any way to write the schema for the following Typescript types. This is what I've tried:

```js
import Joi from "@hapi/joi"

type Bar1 = {}
type Bar2 = {}
type Bar = Bar1 | Bar2
type Contents = {}
type Foo = Bar & Contents

const Bar1 = Joi.object()
const Bar2 = Joi.object()
const Bar = Joi.alternatives().try(Bar1, Bar2)
const Contents = Joi.object()
const Foo = Bar.concat(Contents) // compile error
```

It says

> Argument of type 'ObjectSchema' is not assignable to parameter of type 'AlternativesSchema'.

Note I need to have 5 separate schemas, I cannot write one big schema mixing up all 5 types

Do you have any advice on how to approach the problem?