Implement trivial example endpoint
Namespace articles controller into "V1"

* We're introducing versioning to our API, so as a first step we'll
  namespace the existing endpoint, but without affecting URI structure
Extract "request_spec_helper"

* Used for specs that require ALL the rails things
* This allows us to omit the entire stack for isolated specs :+1:
Implement custom constraint for api routes

* Allows us to constrain routes to requests for paritcular endpoint
  versions
Add constraint to v1 routes
Wrap v1 spec in context
Implement v2 articles endpoint

* Wraps articles in root "articles" node
Extract helper method for versioned api routes
Add real readme! :sparkles:
Link readme to the Good Parts™
Register custom vendor media type for app
Use custom media type in responses
Allow requests with ".json" suffix

* Rails honors file format suffixes over the request's "accept" header
  to specify the requested media type. To allow requests for vendor
  media types while including the format suffix, we must loosen the
  response format for our controller. This is generally fine, but one
  minor side effect is that the "content type" of the response will be
  "application/json" instead of "application/vnd.articles+json". In
  general this probably isn't a big problem.