Add support for Firefox capabilities like for Chrome

[mvorisek]

see

MinkSelenium2Driver/src/Selenium2Driver.php

Line 134 in cd7f67e

$chromeOptions = (isset($desiredCapabilities['goog:chromeOptions']) && is_array($desiredCapabilities['goog:chromeOptions']))? $desiredCapabilities['goog:chromeOptions']:array();

currently goog:chromeOptions are supported for chrome, but similar moz:firefoxOptions options for Firefox are not supported yet

ref: https://developer.mozilla.org/en-US/docs/Web/WebDriver/Capabilities/firefoxOptions

This is a feature request to provide exactly the same support for Firefox

[aik099]

OK. Please send a PR. Also, PR to Behat/MinkExtension is needed to ensure, that Behat can read these compatibilities from a config.