refine comments event control

Hi,

Currently, og_notifications can automatically subscribe a user to notifications when they join a group. The user will then be notified of any new group posts. The user will also receive a notification if someone comments on one of these group posts. The latter is generally undesirable, as new posts may be low-volume, but comments on them may be high volume (at least in my implementation).

I suspect that this behaviour is inherited from the "content type" subscriptions functionality?

I do realize that one can disable the "Node comment" event in the Notification Settings, but it's reasonable to assume that one will still want comment notifications.

An ideal implementation would be one where you could subscribe to a (og) content type, receive notifications about new content, but _only_ receive notifications about comments on that content if you've explicitly subscribed to the thread.

thoughts?