Use feed item count as fingerprint maxlen

I kid you not, <http://blog.bignerdranch.com/feed/> has ~700 entries.

So, new strategy: Set the maxlen to however many items were found in
the initial check of the feed.

I've never seen a feed change that number, so it should be safe.

Worst case scenario is we provide like a 100 item minimum here.
Combine timestamp formatting functions