usage:

new Draggable(wrapper,view,holderClassName,cb);

wrapper:

The HTML Element of wrapper.
structure like this
<div id="wrapper">
  <div>1</div>
  <div>2</div>
  <div>3</div>
  ...
  <div>n</div>
</div>

view:
The wrapper of list which can scroll. default: wrapper

holderClassName:
The className of element in the listItem which can be hold to drag. default: 'holder'

cb: draggable => void
The callback triggered after drop. default: a function to drop listItem to correct direction.

Draggable:
    wrapper: the same as 'wrapper' argument in constructor
    dragElem: the listItem be dragged
    firstIndex: the index of dragged element at first time
    currentIndex: the index of dropping position
    terminate(): revoke listen functions of Draggable
    ...