Compatibility with RFC5005

[joachimesque]

RFC5005:

This specification defines three types of syndicated Web feeds that
enable publication of entries across one or more feed documents.
This includes "paged" feeds for piecemeal access, "archived" feeds
that allow reconstruction of the feed's contents, and feeds that are
explicitly "complete".

The RFC5005 allows for paginated feeds. The system works with <link rel="next"> and <link rel="previous">.

Importing:

I deleted the thing when I didn't think it was too useful: 5a0e262#diff-1a7f54f946253699058ea7b6e0a3ffc1L134

Exporting:

The pagination part should not be too hard to implement, and the start_at:ß option already exists.