Added proxy generic settings

This adds the possibility to run behind a proxy. For instance : 

```yml
default:
    extensions:
        WebDriver\Behat\WebDriverExtension\Extension:
            url: http://localhost:4444/wd/hub
            base_url: http://localhost:8000/
            browser: chrome
            proxy: localhost:8001 # A proxy that will be set in the browser settings.
```