Override cloudinary.yml with environment variables

Allow setting configuration in cloudinary.yml with env vars taking precedence.

This way, one can have some settings in `cloudinary.yml`, e.g.:

```yaml
production:
  enhance_image_tag: true
```

While still passing other (instance-specific) settings in environment variables, e.g. `CLOUDINARY_URL`, as is the cloud way (Heroku, Dokku, etc).