Handle leadfoot-specific capabilities in desiredCapabilities

[jason0x43]

In Leadfoot 2.0.x, leadfoot-specific values in the desiredCapabilities object, such as brokenMouseEvents, are sent to Selenium but not maintained afterwords, at least not by all versions of Selenium or WebDrivers. This means that if a user specifies brokenMouseEvents: true as a capability, it will likely be ignored. Ideally, if a user passes in such a capability, leadfoot should add it to the final capabilities list.

Possibly this should be done through a leadfoot object in the capabilities list so that leadfoot can separate capabilities requested of leadfoot from those requested of the webdriver/selenium.