Add env to the kernelspec __init__

the env attribute was added to the kernelspec and the kernel.json file, but has an error when you try to use it because it's not in `__init__`
The alternative is to build a kwargs dict, and only add to it `if env is not None`. I don't think it matters much which you use.

Adding a 

```python
def _codemirror_mode_default(self):
    return self.language
```

would obviate the need for `__init__` altogether on this class.