Limit items in feed

The all categories atom feed is gigantic and not parseable in some 
feed readers (see [1]). By default Pelican doesn't limit the number
of items in the feed, so I set FEED_MAX_ITEMS (10 seems a good number)
in the settings file.

[1] https://getsatisfaction.com/newsblur/topics/broken_feed_http_ivory_idyll_org_blog_feeds_all_atom_xml