Update Cloudinary widget so that you can opt to inset the filepath of the image, not the filename alone

**Is your feature request related to a problem? Please describe.**
We are going to be integrating Cloudinary with NetlifyCMS. Ideally, when people are adding images via Cloudinary, we would like the resulting markdown to specify the filepath for the file within Cloudinary. At the moment the only options are to output the full Cloudinary URL, or the filename only. Filename only is no use to us as we have different folders so need to know the full path of the image in cloudinary.

**Describe the solution you'd like**
You can opt, when using Cloudinary, for a "filepath only" option.

**Describe alternatives you've considered**
Using the full url and using a preSave hook to strip out everything before the filepath within Cloudinary - but given a number of fields use the image widget and images are also put in the body of content, this doesn't actually seem like it would be very easy to do.

**Additional context**