Added draggable header

[igor-slotin]

Hi!
I've tried the solution for scroll view inside sliding up view, and unfortunately, it didn't work for me.
I've created a method which allows you to declare custom render function for a header and will send panHandlers to it.

for using it you need to declare some rendering method like
renderHeader = (panHendler) => { return ( <View {...panHendler}/> ); }

and set up it to sliding up panel
<SlidingUpPanel visible={this.props.visible} renderDraggableHeader={this.renderHeader} />

If it would be useful for this module - I will create a demo and attach it to readme.