Add support to specify different approaches to select API version

How to put in the specification instructions about how to choose the API version to use? Eg:

* URL path like `/v1`, `/v2`
* URL query string like `?api=v1`, `?api=v2`
* Custom header like `X-API-Version: 1`, `X-API-Version: 2`
* Content-Type/Accept vendor mime-type like `Content-Type: application/vnd.myapi.v2+json`
* others?
