Make better config helper

[Mohammadreza-73]

Modify config helper function to get whole array or a specific key of array:

config('database');             // get whole array
config('database.host');    // get host key from array

[Mohammadreza-73]

Implement config as:

config($name, $key = null);

set second parameter to get array keys.
if second parameter is null return whole config array.