Implement is_regression plugin check

Add a plugin call, "is_regression", called on new events when
the group's status is STATUS_RESOLVED. The call may return
True if this event is a regression, False if it is not, None
to defer to other plugins.

For example, a plugin may wish to examine a tag's values on a
group and only consider the event a regression if it is
contributing a new tag value.

Per discussion #1258.