Support for authenticated proxies

From what I see this gem supports only proxies that don't require Authorization header to be set:

```
    def initialize(proxy_addr, proxy_port)
      timeout = ProxyFetcher.config.proxy_validation_timeout

      @http = HTTP.follow.via(proxy_addr, proxy_port.to_i).timeout(connect: timeout, read: timeout)
    end
```

it would be nice if there was such possibility