Implemented an --ignore-file option to lint/report commands to ignore specific errors

[lobocv]

The ignore-file is a YAML file that lists the paths for each rule for which to ignore errors for.
It can be used when introducing new linting rules which require breaking changes to existing production APIs.

In the example below, say I want to add a lint rule that new URLs must have a version in the path. This would break existing URLs which do not have versions in their path. We can use the ignorefile to grandfather those APIs while enforcing new APIs start using versioning.

Example ignorefile.yml using the burgershop API in the tests:

url-starts-with-major-version:
  - $.paths['/burgers']
  - $.paths['/burgers/{burgerId}']
  - $.paths['/burgers/{burgerId}/dressings']
  - $.paths['/dressings/{dressingId}']
  - $.paths['/dressings']

[daveshanley]

This looks good to me, I really like this implementation. It keeps the spec free of custom extensions, and allows per path configuration.

[daveshanley]

I want to upgrade libopenapi and other core components, so the next release will be a minor one. Would you mind adding some content to the readme about using this? I can then use it for the main docs.