per-index atom feeds

[kkga]

/feed.xml, /blog/feed.xml, etc

[almereyda]

There are three minor validation errors on /feed.xml right now:

https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fter.deno.dev%2F

Will it be hard to implement the per-index feeds, or would you be open for external contributions on this one?