feat(oas2): add format link

[marcofriso]

Closes APIARY-6105

[marcofriso]

I had errors on the same tests locally today after I came back to this branch, I re-ran env GENERATE=1 npm test and now everything is passing again (locally).

The error was:

(node:42618) UnhandledPromiseRejectionWarning: AssertionError: expected { Object (element, meta, ...) } to deeply equal { Object (element, content) }
    at fury.parse (/Users/marcofriso/Coding/Apiary/api-elements-js/packages/openapi2-parser/test/adapter-test.js:52:52)
    at promise.then.result (/Users/marcofriso/Coding/Apiary/api-elements-js/packages/openapi2-parser/node_modules/@apielements/core/lib/fury.js:240:30)
    at process._tickCallback (internal/process/next_tick.js:68:7)
(node:42618) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)

[kylef]

@marcofriso you need to commit all of the change fixtures, including the ones in packages/swagger-zoo.

[marcofriso]

@kylef I re-installed everything with yarn and regenerated the fixtures, now I see the changes in swagger-zoo