Recognizing the hyper-schema meta-schema?

Similar to #28, would it be possible to automatically recognize http://json-schema.org/draft-04/hyper-schema and http://json-schema.org/draft-06/hyper-schema as built-in supported meta-schemas?

```CONSOLE
(venv) HenrysMacBookPro:schemas henry$ ajv compile -s common/user.json
schema common/user.json is invalid
error: no schema with key or ref "http://json-schema.org/draft-04/hyper-schema#"
(venv) HenrysMacBookPro:schemas henry$ curl http://json-schema.org/draft-04/hyper-schema > hyper-schema
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 4639 100 4639 0 0 21803 0 --:--:-- --:--:-- --:--:-- 21882
(venv) HenrysMacBookPro:schemas henry$ ajv compile -m hyper-schema -s common/user.json
schema common/user.json is valid
(venv) HenrysMacBookPro:schemas henry$
```