Add { name: ip, version: 6 } support

We currently ignore version specification for IP. The user can test IPv6, but they do so via any IPv6 addresses they pass in on the command line, not via YAML specification.

We should add support for YAML-specified IPv6 (and v4, of course) version. Verify we produce an error if the YAML file specifies a v6 version, but no v6 address is provided.