Adding the ability to specify a field to use as the CloudinaryImage public_id

Cloudinary has the ability to specify a public ID for uploaded images/files.

I propose adding a field option to the CloudinaryImage field type called `publicID` that will contain the name of a field to be used as the public_id. The option is ignored if the specified field does not exist.

Here's an example:

    List.add({
        slug: { type: Types.Key },
        photo: { type: Types.CloudinaryImage, publicID: 'slug' }
    });