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.

EDIT: The reason I'm forcing the .jpg extension is for simplicity and clearness, mostly. Otherwise, a database would have to be used to match the file with its preview image, which would add unnecessary overhead in my opinion. Users can simply use 'jpg' regardless of the file format instead, considering w3mimgpreview doesn't care about the extension as long as it's actually a supported image.
We could also remove the file extension altogether if that's preferable.

In both cases, users will have to handle programs that force e.g. always append their own extension themselves. pdftoppm always appends '.jpg' for example, so using it must be done this way:
````pdftoppm -f 1 -l 1 -scale-to-x 1366 -singlefile -jpeg -tiffcompression jpeg "$path" "${cached%.*}" && exit 6 || exit 1;;````
A bit uglier if the path is provided without an extension to scope.sh:
````pdftoppm -f 1 -l 1 -scale-to-x 1366 -singlefile -jpeg -tiffcompression jpeg "$path" "$cached" && mv "$cached" "${cached%.*}" && exit 6 || exit 1;;````

Not sure which is the best approach. :)


EDIT2: Also, ranger tries to preview all images using w3mimgdisplay first. Even PSD files, for examples, which will fail silently. A workaround would be to check for the mimetype before drawing the image. A quick example, in browsercolumns.py:
````
        some_list = ["image/jpg", "image/png", …]
        if self.fm.settings.preview_images and self.target.image *and self.target._mimetype in some_list*:
            self.set_image(self.target.realpath)
            Pager.draw(self)
        else:
            ...
````
You could also abandon the ````self.target.image```` check completely and always run scope.sh (or scope_images.sh if you want to separate the preview files)… then scope.sh/scope.sh_images would directly return ````$path```` if it's an image, and would allow the user to override whatever they'd like to.