add key for rendering items

it will increase speed of VNode-Dom comparison and patch
for example for list of images when new slots added to the list only these images will requested (at this moment all 30 will be requested)

Author: linniksa

src/index.js

@@ -269,6 +269,7 @@ const VirtualList = Vue.component('virtual-list', {
         if (dataSource) {
           if (Object.prototype.hasOwnProperty.call(dataSource, dataKey)) {
             slots.push(h(Item, {
+              key: dataSource[dataKey],
               props: {
                 index,
                 tag: itemTag,