PUT new attribute backwards compatibility optional

[davidboden]

#136 made the PUT request incompatibility check stricter, considering an additional field as a breaking change. This makes sense for a scenario where updates are taking place, but is too strict if PUTs represent only inserts.

What would be the best way of optionally removing the strict handling of PUTs? Options could include:

• Surfacing more information about how the changedOperation.isIncompatible() decision was made, allowing the user of the OpenApiDiff to use more granular information to decide whether it should be considered as a breaking change.
• Adding a command-line flag to remove the PUT strictness for all cases in a run.
• Writing some code to traverse the object graph for PUTs when the diff has completed and decide whether the strict PUT handling is what caused the incompatibility, and then ignore it.