Mark all items read when the fever group_id is 0

Stringer is not currently marking all items as read when the `group_id` is 0. The Fever API docs state:

>You can mark the “Kindling” super group (and the “Sparks” super group) as read by adding the following four arguments to your POST data:
>
>mark=group
>as=read
>id=0
>before=? where ? is replaced with the Unix timestamp of the the local client’s last items API request

Stringer has no notion of kindling, in this case it's safe to mark all items read.