Add maintenance bits to git

It's somewhat local to my setup but it's handy to have in git for future
sanity.
Simplify parse_config

It was pulling in the JSON in a rather unidiomatic fashion. Having
simplified it's obvious now that's just pull data out of a JSON file.
Don't pretend we won't get a Redis instance

Either an exception is thrown or we get back a Redis instance, there's
no way we're getting an undef back.
Add subreddit to link output

By way of a big ol' slightly unnecessary refactoring. Now lists are
frobbed in the large and we maintain as little state as necessary while
leaving code coherent.

Also have add_seen take a list of ids given that's its common usage.