Add support for chromeOptions argument.

Hello, 

The command-line options `chrome.switches`, `chrome.extensions` and `chrome.binary` are not supported anymore by latest versions of ChromeDriver (see [Capabilities (aka ChromeOptions)](https://sites.google.com/a/chromium.org/chromedriver/capabilities)). 

Tested using Chrome 35 and ChromeDriver 2.10.

I noticed that while trying to set the `--test-type` flag, to avoid a warning displayed when Chrome is launched with the `--ignore-certificate-errors` flag. 

The following configuration was not working: 

```
extensions:
        Behat\MinkExtension\Extension:
            selenium2:
                capabilities:
                    chrome:
                        switches: ["--start-maximized", "--test-type"]
```

My change is backward compatible because it just adds a `chromeOptions` key to the desired capabilities. Other `chrome.*` keys are just ignored so it's fine. 

This is actually a fix, as to be more flexible it would be nice to support all the keys of ChromeOptions, but this also needs a change in MinkExtension. 

__Related issues__:

* #174