Added option to add thumbnail

[Ovi]

We can now provide an array of objects in order to supply thumbnail of the images:

<FbImageLibrary images={[
  { url: 'imageURL', thumbnail: 'imageThumbnailURL' },
  { url: 'imageURL', thumbnail: 'imageThumbnailURL' },
  { url: 'imageURL', thumbnail: 'imageThumbnailURL' },
]}

thumbnail will fallback to url

However, if we just want to show images we can pass them as an array of strings:

<FbImageLibrary images={[
  'imageURL',
  'imageURL',
  'imageURL',
]}

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Resolves #11