Adds is_confirmed param to BP_Group_Member_Query, defaulting to true

This additional parameter ensures that BP_Group_Member_Query distinguishes
between confirmed and unconfirmed members when pulling up lists of group
members.

Fixes #5072

Props Mike_Cowobo

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7237 cdf35c40-ae34-48e0-9cc9-0c9da1808c22