how to init current-index

```
<slick class="slider single-item"
               dots=true
               current-index="index"
               responsive="breakpoints"
               slides-to-show=1
               slides-to-scroll=1
               prev-arrow='.slick-prev'
               next-arrow='.slick-next'></slick>
```
here is the code,but i find that the current-index failed.
