Explore Joi 9 custom rules feature

[jonathanglasmeyer]

Joi 9 is around the corner and it has an important feature for us: it allows to write custom rules. This is really cool because it probably means that we can implement all validation with joi, also the more complex ones suggested by @bebraw.
Have a look here at the new API documentation for the Joi.extend() feature:
hapijs/joi@8f0ab60

I'm really excited to try this, will probably install the joi beta 9 version to test this sometime this weekend.

[jonathanglasmeyer]

Done via #81. Works great. :)