Adding the ability to specify a folder for a CloudinaryImage

Cloudinary has the ability to specify a folder to organize uploaded images. To enable the use of Cloudinary folders I added a Keystone option called `cloudinary folders` that should be set to `true`.

    Keystone.init({
        ...
        'cloudinary folders': true
        ...
    )};

or

    Keystone.set('cloudinary folders', true);

A path per CloudinaryImage field can be specified using the `folder` option.

    List.add({
        ...
        photo: { type: Types.CloudinaryImage, folder: 'path/to/image' },
        ....
    });

Otherwise default folder path will be generated from the `cloudinary prefix`, the `list path` and `field path`. 

    prefix/list/field

or

    list/field

This creates `prefix` as a global variable, not a local one.