Limit max feed items on pelican.conf.py

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 using FEED_MAX_ITEMS (10 seems a good number).

[1] https://getsatisfaction.com/newsblur/topics/broken_feed_http_ivory_idyll_org_blog_feeds_all_atom_xml