custom validators

#### `any.custom(fn)`

Allows a custom validation function.
- `fn` - function which takes parameter `value` and returns either string error message or null.

```javascript
var schema = {
    primeNumber: Joi.number().custom(function (n) {
        if (!isPrime(n)) {
            return 'must be prime';
        }
        return null;
    })
};