Support for update an already existing media.

[damarev]

It would be a nice to have funtionality to update an already existing Media with a fresh new file/image.

Right now, through Optix\Media\MediaUploader you can only create a Media model from an UploadedFile through the upload() method.

Maybe something like this would do the trick?

public function upload($existingMedia = null)
{
    $model = config('media.model');

    $media = $existingMedia ?: new $model();

    //...
}