Support array `images` for featured image

[budparr]

Right now the featured image is a string, but Hugo's embedded templates look for an array of images.

Users have solved the problem by using Page Resources which both Hugo and Ananke support. But we should probably look for an array of images on top of our featured_image string.

[regisphilibert]

This should take care of #161

[nedjo]

How would image array data be structured in page front matter? Would existing sites have to update their featured_image values? Can we harmonize a fix with changes proposed in https://github.com/theNewDynamic/gohugo-theme-ananke/pull/540, specifically the featured_image_data property?