Feature request: Atom 1.0 feeds

[ChillyCider]

I think Atom is pretty popular, so it might be a cool feature. Maybe enabled on a per-collection basis (#368).

[epage]

Looks like there is https://crates.io/crates/atom_syndication that we can use.

We'd probably support it like rss and jsonfeed.

[ChillyCider]

I'd be interested in trying my hand at implementing this. Never tried Rust before, but now's as good a time as any. If there are objections lemme know

[epage]

Go for it! The rss and jsonfeed implementations should provide a pretty good starting point for this.