add image diff to changelog
    
    this is a port of something currently done in my 'check-compose'
    tool which produces the 'compose check report' emails. I think
    it makes sense to do it here instead. It's pretty simple: we
    consider the 3-tuple (subvariant, type, arch) for an image to
    be a unique identifier for the image, and we compare the images
    in each compose and produce 'added_images' and 'dropped_images'
    from sets of the identifiers. We use the same identifier run
    through " ".join() as the human-readable identifier and include
    just a count of added/removed images in the summary, the id for
    each added and removed image in the brief log, and the id plus
    the path for each added and removed image in the verbose log.
    
    I tried to future proof it a bit, in case we need to change the
    identifier format at any point, the idea is that we can just
    change the `image_id` method and the rest of the code should
    keep working. At first I just got the tuples and stored those
    as added_images and dropped_images, but then it's a bit messy
    in future if we change the identifier format. It's also set up
    such that we can also add check-compose's 'expected images'
    feature quite easily using the `get_images` dict (planning that
    as a separate PR).
    
    Fixes #3
    Fixes #10