Add support for Caption Descriptor

The Caption Descriptor is a referenced property of the Gallery Item referenced property, which is a property of Gallery/Mosaic

https://developer.apple.com/library/content/documentation/General/Conceptual/Apple_News_Format_Ref/CaptionDescriptor.html#//apple_ref/doc/uid/TP40015408-CH91-SW1

It allows you to include HTML/markdown in a caption. This task is to support it.