Initial support for OpenAPI 3.0

[sprootshift]

This pull request adds initial support for OpenAPI 3.0.

enjoi dependency is a placeholder to be updated later if/when tlivings/enjoi#92 is merged.

[tlivings]

Also have #183 PR...

[sprootshift]

My understanding is that #181 is a very different approach. It is more of a refactoring of the library to abstract away the details of Swagger and OpenAPI3 schema into a single proxy object. I am not sure how much of it is really needed, given that swagger-parser does a decent job parsing both. In my approach I am working directly with swagger-parser output and it seems to work reasonably well. But if you think there is value in the intermediate proxy representation of swagger-parser output, I think it can be merged into this PR.

[sprootshift]

@tlivings So what's your view of these PRs? Is it something you consider to merge?

[tlivings]

I am trying to get to this today. Been slammed and distracted by day job.

[sprootshift]

Thanks for merging it. Could you please publish enjoi first and then this, after I update dependency to npm enjoi?