Add an option to write failed/changed snapshots to disk

[bradleyjkemp]

Debugging large snapshot changes in the terminal is difficult. #64 could help but you might still want to view the two files side by side.

Suggested solution is adding a new flag/option that writes failing snapshots to a file with an extra suffix e.g. .failed

Requested by @thecodejunkie: https://twitter.com/TheCodeJunkie/status/1388797594521022464