sub query in select
a workaround for now is

```
Game.select(Game, fn.Sum(Stat.viewers).alias('viewers')).join(Stat, JOIN_LEFT_OUTER).group_by(Game).order_by(R('viewers DESC')).limit(5) 
```

but that runs up to 10 times slower then the sub queries on

is there a way for peewee to support sub queries like this one

SELECT g.*,
(SELECT SUM(s.viewers) FROM stats s WHERE s.game_id = g.id) AS viewers
FROM games AS g
WHERE g.id != 96
ORDER BY viewers DESC
LIMIT 5
