enable limiting size of exported feed
Created by: rjw57
JWPlatform no longer support importing feeds of greater than 1000 items. Teach genmrss how to truncate the list of items it uses to form the feed. When merged, the generated MRSS feed should now be limited to 1000 items.
We may want to generate 2 (or 3) feeds for the last 0-1000, 1000-2000, ... modified items to make sure we catch everything.