Feature request: provide a diff of changed resources

A common use case we have for `iamy` is to add a user to the assume-role-policy-document for some role. When there are already a number of users in such a list, it can be tricky to spot the change shown when `iamy push` shows the command it's going to run.

It would be super handy if there was an (optional?) way to show a diff view of the change between what's live and what's about to be pushed, so you could easily see (for example) the one user being added to the policy. This would no doubt be helpful on selected other resources too.

Will try and come up with some scrubbed example output here shortly.