Added flags support for mysqli::real_connect in Mysqli driver.

This is necessary if you want to set connection options like compression or SSL encryption.

Recently I wanted to use DBAL with Mysqli driver and I noticed that there is no possibility to enable connection compression, which is done in mysqli by setting flags to mysqli::real_connect. DBAL Mysqli driver lacks support for setting any flag, so I decided to propose my change to add such possibility. 

With my changes you can set flags in that way:

```php
$connectionParams = array(
    'dbname' => 'mydb',
    'user' => 'user',
    'password' => 'secret',
    'host' => 'localhost',
    'driverClass' => 'Doctrine\DBAL\Driver\Mysqli\Driver',
    'driverOptions' => array(
        'flags' => MYSQLI_CLIENT_COMPRESS
    )
);
```