Adds generator/summary/favicon fields

[zachleat]

This is an upstream issue for the work @DougReeder did on eleventy-base-blog before it moved into a Virtual Template approach.

This work could be adapted in the virtual template too: 11ty/eleventy-base-blog#180

[zachleat]

https://github.com/11ty/eleventy-plugin-rss/blob/a639e447d27edcc91fdc5d05b5453bd0c8b05b29/src/virtualTemplate.js is where this would go now