Adding CloudinaryImages folders

Implemented `folder` option in `CloudinaryImages` (and some docs to go along it). This is similar the the implementation in #604, but aligned with our original implementation in the `CloudinaryImage` field.

Just like before, it is enabled globally using:

```js
keystone.set('cloudinary folders', true);
```

and the default folder can be overridden on a per-field basis using:

```js
List.add({
  images: { type: Types.CloudinaryImages, folder: 'path/to/image' }
});
```