How to validate a number is even?

#### Support plan

* *is this issue currently blocking your project?* (yes/no):
No
* *is this issue affecting a production system?* (yes/no):
No

#### Context

* *node version*: 12
* *module version*: 6
* *environment* (e.g. node, browser, native): Node
* *used with* (e.g. hapi application, another framework, standalone, ...): Standalone
* *any other relevant information*:

#### How can we help?

I have a use case that requires numeric values to be even. Values are entered via an API request, so I can not use the UI to enforce the numbers. Is there any feature within Joi that would allow me to validate a number is even?