Add support for customising phantom command line options

Currently phantomjs gets created with these capabilities:

``` js
Selenium::WebDriver::Remote::Capabilities.phantomjs({
  'phantomjs.cli.args' => ['--ssl-protocol=any', '--web-security=false', '--ignore-ssl-errors=true', '--load-images=false']
})
```

I do not see a way to enable --load-images.
