Add archivebox integration.

[JeckDev]

• I have read this document: https://miniflux.app/opinionated.html#feature-request

The upcoming archivebox release has REST api support, which makes it an excellent candidate for an integration. See: ArchiveBox/ArchiveBox#496 (comment)

I'm writing a feature request here both as a reminder to myself, to check people's thoughts, and on the off chance that someone else might get around to it before I do (because I'm unlikely to have free time this month).

[jvoisin]

What's the point? Miniflux is already saving the post's content.
I think this feature doesn't fit into miniflux' scope, and would be better in an rss2archive.py script or whatever.