Support for OpenAPI3

[PornthipSaechong]

Hi, is there a consideration to extend this library to support openapi3. Currently, swagger-parser is now v9 and support openapi3 destructuring. Is there an issue with using newer swagger-parser to parse openapi3 doc before using the existing comparison engine? I have tried with the upgraded swagger-parser and there is no error. My only worry is that the comparison engine is not catered for openapi3 and that there are missing component that we need to add on to the comparison engine. Btw, great job on coming up with this package, it's really useful 👍