OpenAPI specification input

[james-blitzm]

This project seems like a good candidate to implement using an OpenAPI-standard specification as an input schema for generation of Singer models.

i.e. Taking the JSON generation one step further to make use of an officially published OpenAPI specification for the target source API.

For example, here is the official OpenAPI specification for the Stripe API v3: https://raw.githubusercontent.com/stripe/openapi/master/openapi/spec3.yaml