Allow ignoreZoomSetting in config

Hello,

As noted in #41 the Selenium2 IEDriver can take an option called `ignoreZoomSetting`. However the config validation of the extension doesn't allows it.

This PR add support for such option.

This option is really useful when applying Behat/Mink goodness to webapp using the zoom level, typically webapps using ActiveX related stuff (yes, there still are some :))