Image Editing features

Imagine you want to use exported ppt slides in your docs, but you don't want to always show the header and footer of the slides which might containt the slide number and other details.
This disturbs the look and feel of your docs.

Wouldn't it be nice if you could crop those images on the fly?
Something like `image::slide3.png[crop="10%,0,5%,0"]?

This looks like a change to asciidoc or asciidoctor extension, but asciidoctor does not change images or create new images.
So a different mechanism would be needed - sounds like a feature for docToolchain, doesn't it?

Here are some ideas which come to my mind:

- crop: just crop the image given the amount to crop as pixels or percentage. top, right, bottom, left
- spot: a spotlight effect like the one created by keynote or the logitech presenter:
- frame: insert the image into a frame. This can be used to add a frame of a mobile device or just to add a shadow
- overlay: add something like a watermark

This feature could be implemented

- as part of the pptExport. But then it wouldn't be universally usable
- as part of the adoc-file. Add seomthing as comment which will be parsed by docToolchain. dT will then create the images upfront and the doc can expect them to be available. Problem: dT will not know about the `imagesDir`