Generate responsive images

Cloudinary supports generating responsive images.

https://cloudinary.com/documentation/responsive_images#responsive_breakpoint_generator

```
cloudinary.uploader.v2.upload("sample.jpg", { responsive_breakpoints: { create_derived: true, bytes_step: 20000, min_width: 200, max_width: 1000, transformation: { crop: 'fill', aspect_ratio: '16:9', gravity: 'auto' } } }, function(error, result) { console.log(result); });
```

The sizes used should be configurable in the Netlify file-based configuration. Ideally we could also target configuration, for instance, using element selectors to use specific sizes for that specific element(s).

For example: All images get `sizes="..."` while `.my-images` get `sizes="..."`

### More Info
- https://cloudinary.com/blog/introducing_intelligent_responsive_image_breakpoints_solutions
- https://www.responsivebreakpoints.com/