Capability of adding "test" op in compare result

[stakira]

Being able to optionally add a "test" operation before "replace" and "remove" can be useful in certain scenarios. For example, it can be used to validate a serial of patches, and can be very helpful when it comes to json data change visualization.

Looking at the code, it seems easy enough to be done. I can make a PR if it sounds like a good idea. The remaining question is how would you like the switch being provided and how would you like it named.