Get `feature_image_width` and `feature_image_height` via Content API

At the moment we can get dimensions of images inside HTML (from attributes) but not dimensions of the feature image. The feature_image_caption and feature_image_alt fields have been added recently, it would be nice to add feature_image_width and feature_image_height as well.

Also, Ghost themes automatically add JSON-LD with feature image width and height, we can’t do that ourselves using Content API data.