Allow defining Enums ahead of time

[dbanty]

Because Enum isn't a real schema type in OpenAPI, we currently create them by making up a name from the property that defined them. That means if you have the same Enum in multiple places with different property names, you'll get multiple enums. Member names are also made up from the values, so those are sometimes not very descriptive.

We should add a section to config.yml that lets users define Enums, then check any Enum we come across to see if its values match the values of a known Enum. If they do, use that Enum.