Exclude directories from diff threshold check

[Atreb92]

Hi! In my use case I use my storage as a backup target for configuration snapshots which rotate. This results in many files (correctly) being removed from old snapshots. The snapraid-runner detects these removals and stops execution since they exceed the defined threshold. Instead of abritrary increasing the threshold in order to avoid this, I implemented a filter which allows the user to define paths which should not be considered in the diff count for verifying the threshold.