Added 'config_exists' and 'config_key_exists' to validation plugins

Usefull if you have a 'huge' config file and want to check if a value exists or if is a valid array key in a config file. It's an alternative to 'in:foo,bar,baz'.

## Eg 1.: config file "brazil_states_ acronym.php"
return 
[
   'ac',
   'ba',
   'ce',
   'es',
    ... more 23 itens
] ;
$validator = $this->validator->create($this->request->post(),
[
        "state" => "required|config_exists:brazil_states_ acronym",
]);


## Eg 2: config file "oficial_bank_codes.php"
return 
[
    '067' => 'Bank Foo',
    '485' => 'Bank Bar ',
    '723' => 'Bank Hello World',
    '126' => 'Bank Example 1',
    '459' => 'Bank Example 2',
    ... more 127 itens
] ;
$validator = $this->validator->create($this->request->post(),
[
        "bank_code" => "required|config_key_exists:oficial_bank_codes",
]);