Have feed lists be flat
Add docstrings to feed.py
Clean up comments
Clean up Feed.check()
Move new item info up
Bump min interval to 60 seconds
format_timestamp: optionally convert to local time
Write updates to output JSON files
Generate items list timestamp descending
Rename initial to initial_limit
Sort the feed list on import

This way the feeds aren't checked in the same order each time river is
launched.
Don't write to index.json, just archive JSON
Sort the new items timestamp descending
Track previously seen items in a set, not a list
Implement __hash__ so self.items works as a set
Only add virtual timestamp if it extends update interval
Remove dupe feeds
Handle feeds that fail from the start
Capture warnings (and above) to /tmp/river.log