wish: support iframe backgrounds

[markstos]

It would be nice to support interactive maps embedded as section backgrounds that are hosted elsewhere.

Perhaps these could be implemented with <section data-iframe="https://some-map-url-to-embed.com"> and then about there in the code, support could be for them as section backgrounds:

storymap/dist/storymap.2.5.js

Line 247 in e4cade0

$.each(sections, function (key, element) {

For example, a lot of Esri / ArcGIS Online maps allow iframe embedding.