Add support for vendor extensions about swagger-diff

[Sayi]

I have not used vendor extensions, you will do better than me.

[egoodhall]

Hi again, I've made a pull request that adds support for vendor extension diffing. It doesn't make any breaking changes to the API, so it shouldn't break any existing code. It does add overloaded compareV1 and compareV2 methods, with an additional boolean argument withExtensions that will enable vendor extension diffing if set to true. I'd appreciate if you could take a look at it when you get the chance, and let me know if there's anything I can do to help get it merged/deployed