Add size differ

[nkubala]

It would be interesting to have a --type=size diff, which just computes the size difference of two images. This could help as a sanity test in things like CI where users expect images NOT to change dramatically, but don't want to parse the output of the package differs.

[peter-evans]

@nkubala This issue can be closed. Apologies. I should have included Fixes #236 in the PR.