It should be possible to control the number of items included in a feed
independently of the number of items included on the page (the latter,
however, possibly setting an upper limit). This would be particularly
useful on archive pages providing a feed. Presently the feed grows huge, if
the archive page has no limit on the entries listed on it (as in the list
of [all entries][ionfaq] in the Ion FAQ). An alternative useful filter
would be filtering by the age of the page.

  [ionfaq]: http://iki.fi/tuomov/ion/faq/entries.html

> [[todo/Done]], option name is `feedshow` --[[Joey]]