Need FastImage Preload Promise to know when preload done to download all images

[putuoka]

Need to know when preload done to download all images (depend on internet connection & hosting) either with array or for loops like this

for (let index = 0; index < jsonData.length; index++) {
      //urlOfImages.push({uri: jsonData[index]});
      FastImage.preload([{uri: jsonData[index]}]);
      count = index;
    }

[nicomontanari]

See #437