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

```
_.$prevArrow = $(
    '<button type="button" class="slick-prev">Previous</button>').appendTo( 
    _.$slider);
_.$nextArrow = $(
    '<button type="button" class="slick-next">Next</button>').appendTo(
    _.$slider);
```

I would like to have more control over these arrows. 

I believe either:
1. Having the option of where to append them to
2. 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?