Always save new actual result if snapshot comparison fails

[skuzzle]

If a snapshot assertion fails, we could always persist the new actual result in a sibling file next to the actual .snapshot. Maybe with .snapshot_actual extension.
That would allow to update the snapshot by just removing the old file and removing the _actual suffix which does not require to run the test again

The .snapshot_actual files should not be checked into any SCM to avoid unncessary noise in the Git status overview.

[skuzzle]

Will be released with 1.7.0