Support custom policy

[matankila]

I think there is a much needed feature,
Use a custom policy for the diff with options like

• Exclude breaking change
• Include breaking change

this way an organization can use the product how he sees fit.

First I would like to see if this idea is approved,

file name: pb.yaml (shorthand for pb33f)
Content:

ruleset:
  - “breaking-changes”:
      exclude_operations_with_extension: “x-foo”,
      include_operations_with_extension: “x-bar”

[daveshanley]

other than extensions, what else would you want to configure breaking changes for?