Add ability to use custom direction controls ("prev" / "next")

While flexslider is indeed very "flex"ible I think 95% percent of the people are like me: they want to use it as carousel, they want to style the controls, everything else works as expected, job done.

Customizing the controls however is less streamline then it could be since the markup for the controls are generated by the plugin. I find this ok for the controlNav (the "dots") since the number of dots depend on the number of slides. For the directionNav (prev/next button) however I more often then not came up with (sometimes crazy) solution to position and style the arrows like I wanted them to have.

This PR adds the ability to pass a jQuery obj/selector during initialization, that are simply the two direction buttons (check the example).