Define a JSON type

JSON is such a common interchange format it might make sense to define it as a specific type.

``` python3
JSON = t.Union[str, int, float, bool, None, t.Mapping[str, 'JSON'], t.List['JSON']]
```

Not sure if this should go into `typing` or be introduces as `json.JSONType` instead (or if it's even worth it considering the variability of the type).
