Global 'force' configuration option to always request login credentials.

This implements a common `force` configuration variable that is recognized by major providers in a simple and effective way, so that issue https://github.com/hybridauth/hybridauth/issues/287 can be easier and cleaner.
```php
 'Facebook' => array (
     'enabled' => true,
     'keys'    => array ( 'id' => '65...', 'secret' => 'ae...' ),
     'scope'   => 'email, ...',
     'display' => 'popup',
     'force'   => true
 ),
 'Twitter' => array (
     'enabled' => true,
     'keys'    => array ( 'key' => 'IA...', 'secret' => 'V6...' ),
     'force'   => true
 ),
```
best,
FA