Support HtmlUnitDriver client options

There are options that could be set for HtmlUnitDriver but are not available in Drone configuration.

Drone would need to proxy HtmlUnitDriver and override protected modifyWebClient method because this is not exposed in Selenium.

https://code.google.com/p/selenium/source/browse/java/client/src/org/openqa/selenium/htmlunit/HtmlUnitDriver.java?r=1615e198f0e0b9d7a0cc60bedf475fe23b9c7e1e#266