Provide the action the user performed to trigger a slide change to onBefore and onAfter

[BenSayers]

My use case is I would like to track how users are navigating between slides (swiping, nav button clicks, pagination clicks, custom keyboard shortcuts I added, etc). It would be useful if the action was provided in the object passed to the onBefore and onAfter callbacks. This may also mean calls to flexslider methods that change the slider position should accept a user action that triggered this call.