In the common case of not specifying a size, we are already running
"du" over the image to figure out how big it is. Leverage that by
saving it's output and displaying a pruned list of big files when
requested.
We add a flag to show a summarised option (files >10MiB) and another
to show full output, should you wish that level of detail.
"Invocation" documentation is updated (and formatted a little better
while we're here).
Change-Id: I255800790a62fed1c82fcd311f1cc29c9867766d
fa3c5e3056