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. I was able to use that option, browse to http://localhost:9000 in Chrome, and enter `__run()` to start the tests in debugger (unless specifying autorun option which starts them automatically). But you can't see the tests listed in scripts even if you reload, and if you try to pause execution it doesn't pause until it hits the last js file, which isn't a test. But, 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`, so decided to go through with the PR.

Classifying feature as "experimental", because I think that later it should be replaced with something cleaner that only lets you set options that you would actually use that won't conflict/be confused with other options.

Would be nice to have similar support for Selenium, but for Selenium, the Ruby binding is being used, so a string containing CLI switches for the *driver* would not be correct. Selenium *does* support CLI switches per browser, so maybe it would have `browser_cli_options`, but someone else can do that when it's needed.