Add support for connecting redis using unix socket

This way, if hostname looks like `/var/run/redis/redis.sock` then `Connection` class will use `unix sockets` for connecting redis server.

In config file:

```php
		'redis' => [
			'class' => 'yii\redis\Connection',
			'hostname' => '/var/run/redis/redis.sock',
			'database' => 0,
		],
```