JSON loader

We need a method to load a JSON file with providers and return either a new `Bunch` or append the JSON content to an existing `Bunch`. Behaviour could be:

```python
import xyzservices.providers as xyz
import xyzservices

# Either
new_providers = xyzservices.load_json("my_list.json")

# Or:
xyz.load_json("my_list.json")
```

This should support both local and remote JSON files.