Added 'where' clauses to 'exists' validator

Added possibility to parse 'where' clauses to 'exists' validator.

Eg:

$validator = $this->validator->create($this->request->post(),
[
    "user_id"         => "exists:users,id,activated,=,1,banned,=,0",
    "customer_id" => "exists:customers,id,type,!=,reseller,activated,=,1",
    "admin_id"      => "exists:administrators,id,date_added,>=,1999-12-31,date_added,<=,2000-12-31"
]);


Parse unlimited parameters following the the sequence: 'column','operator','value'