Add Background Image Support

[aurooba]

Often times, content grids have background images. It'd be nice if the plugin supported background images as well. Unsure at this time if this is a feature better put in pro. But for now, proposing it for the free plugin.

[jonschr]

Sample functionality: https://www.visitgreeley.org/ (adventures section)

The components that would enable this would be a link and a background image which attach to each

, where the

is positioned relatively and an inner is absolutely positioned with inset:0 and z-index: 5

This one is pretty well beyond the scope of this plugin, perhaps, but those are the only features necessary to enable layouts like this one.