Add Eta support to Eleventy

[bgub]

The Eleventy static site generator is growing increasingly popular, and it would be great to add support for the Eta template engine. See this issue in the Eleventy repository for the feature request.

https://github.com/11ty/eleventy-plugin-vue looks like a good example we could get started with. It should theoretically be easy to add Eta support, because Eta implements file handling, partials, layouts, and caching on its own.