Do not require DriverType or OptionsType if specifying a custom factory #51
Description
In the configuration, at present, the DriverType is mandatory always, and the OptionsType is mandatory if either DriverType is RemoteWebDriver or it is a non-standard driver for an esoteric browser.
Actually, if DriverFactoryType has been specified, then DriverType and OptionsType are unusued, so they don't need to be mandatory.