Feature request: incrementals

[screwer]

When appending - instead of create recovery file (which anyway doesnt work properly. at least in my experience) produce incremental update file. And another option to roll-in that incremental update over initial image in safe way.

That have two pros:

1. we dont change original image until almost whole work done.
2. we have an update blob, which could be archived and later applied to previous archives at another machine/storage in incremental manner, This will make maintain sqfs images backups much more convenient.