Atom/RSS feed support?

[piegamesde]

Nix customarily uses HTTP redirects to provide download link versioning, but Arch Linux uses proper web feeds https://archlinux.org/feeds/, and the more I think about it the more I like this idea. Though unsure how to deal with feeds that combine multiple packages into one etc.

Possibly this could also subsume other forms of pin types, like GitHub artifacts in #44, by allowing to easily host an RSS bridge from whatever domain-specific layout to the thing npins consumes

[Coca162]

I think there is a lot of potential here if we manage to make this functional for a wide variety of RSS feeds, but the unspecified nature of it for this purpose has me a bit worried for questions like if npins should update via pubDate or using the first item in the list (or for those that need certain semver requirements!).

It would also remove the need for a specific pypi pin if it works well for it. More examples of RSS feeds for things users would pin would be nice too.