scope.sh support for image previews with w3mimgpreview

As per your comments in #102, I think this would be a better solution indeed. It can be used for anything.

I'm not sure if this is an issue, but preview_images is assumed to be set to true. However, since scope.sh does not make use of image previews by default, the user must explicitly 'enable' it still.

Another solution would be to have a separate scope.sh only for image previews.