Add file size to imagelist output
    
    So, I realized today it would be useful for fedfind to know the
    *sizes* of image files. This 'imagelist' file is generated for
    fedfind's convenience - it uses it to quickly find all image
    files in the tree - and AFAIK nothing else uses it, so changing
    its format should be safe. I would take responsibility for
    updating fedfind to handle the new format.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>