adding experimental support for driver_cli_options

Adding experimental support for driver_cli_options to use additional cli options for PhantomJS. Original intent was for this to allow setting `--remote-debugger-port=9000` to allow debugging of tests (though I've not had much luck with that- have to use Chrome and type `__run()` to run unless specifying autorun option, and then if you try to pause execution it doesn't pause until it hits the last js file, which isn't a test). This change provides value though in some of the other features listed [here](https://github.com/ariya/phantomjs/wiki/API-Reference#command-line-options), e.g. `--ssl-certificates-path=/path/to/certs`. Classifying feature as "experimental" in readme, because I think that over time it should be replaced with something cleaner that only lets you set options that you would actually use and that wouldn't chance conflict/confusion with other options. Would be nice to have similar for Selenium, but for Selenium, the Ruby binding is being used, so a string containing CLI switches would not be a direct relation. Selenium does support CLI switches per browser, so maybe it would have `browser_cli_options`, but someone else can figure that out. ;)