Option Request: append arrows to element other than _.$slider

Add appendArrows option.

I would like to have more control over these arrows, and I specifically need to wrap them with a class. I believe either having the option of where to append them to, or being able to wrap them with a class might work just as well. For people that would like to add them in a very different and already-existing area, they may prefer the former.

What do you think?