support webdriver chrome options

I'd propose support for chrome options to be flexible w.r.t. custom flags, e.g.

```json
{
	"browsers": [{
 		"chrome": {
			"chromeOptions": {
    	 		"args": ["enable-experimental-web-platform-features", "js-flags=--harmony"]
			}
		}
	}]
}
```

https://sites.google.com/a/chromium.org/chromedriver/capabilities