No possibility to open cropper with custom file upload

[tkharuk]

Since Cropper is opened when previewImage is available and it can be changed only via internal Uploader there is no way to pass file directly and open Cropper instantly without the need to select file from file system.

https://github.com/react-component/cropping/blob/79334d67a2ffb60996c7100ef05720f4bdb4a128/src/CropViewer.tsx#L166