Add a placeholder property

[frinyvonnick]

We should add the possibility to display a placeholder component if the data isn't loaded yet.

src/index.js line 47

{({ index, style }) => (
  <div style={style}>
-  {children[index]}
+  {children[index] || placeholder}
  </div>
)}

We should verify if children[index] exists otherwise we display a placeholder component passed as prop by the user.

[trybick]

I can try this!