add pg-unfollow command

Unfollow a Heroku Postgres database:

```
$ hk pg-unfollow rose                  
warning: heroku-postgresql-rose on myapp will permanently stop following heroku-postgresql-cyan.
warning: This cannot be undone. Please type "rose" to continue:
> rose
Unfollowed heroku-postgresql-rose on myapp.
```

I also pulled out some shared logic into `mustGetDBInfoAndAddonMap()`, which will likely be applicable to some other `pg-*` commands