[5.3] Allow to hook into the validator instance from the FormRequest

[shadoWalker89]

Before this PR, when i need to add an After Validation Hook in a FormRequest class i must define a validator() method

public function validator(Factory $factory)
{
    $validator = $factory->make($this->all(), $this->rules(), $this->messages(), $this->attributes());

    $validator->after(function($v) {
        if( true ) {
            $v->errors()->add('field', 'Something is wrong with this field!');
        }
    });

    return $validator;
}

The down side is that i need to define how the validator instance is created.

This PR will allow to easily add After Validation Hook to the validator instance without the need to define how the validator instance is made.

class CommentStoreRequest extends FormRequest
{
    protected function validatorHooks($validator)
    {
        $validator->after(function($v) {
            if( true ) {
                $v->errors()->add('field', 'Something is wrong with this field!');
            }
        });
    }
}

[taylorotwell]

You don't actually have to define it in that way... you could just do:

public function validator()
{
     $validator = Validator::make($this->all(), [
          //
     ]);

    // ...

    return $validator;
}

[taylorotwell]

I might call this method withValidator or something like that... not sure exactly.