Add Session method to merge environment settings with per-request settings

See also #2129 
While we're here, these two conditional blocks make me feel a bit uncomfortable. Maybe combine the two? Something like:

```python
if verify is True or verify is None:
    verify = os.environ.get('REQUESTS_CA_BUNDLE') or os.environ.get('CURL_CA_BUNDLE')
```

Obviously there's a line length issue here, but I feel like it's a little cleaner. We'd need to update the commenting too.

I'm not hugely attached to it, what do you think?