notmuch: add nm_exclude_tags

Based on patch from: Raghavendra D Prabhu <rprabhu@wnohang.net>

 This makes use of notmuch_query_add_tag_exclude in notmuch API, to
 exclude messages tagged with tags from nm_exclude_tags from  query
 results by default unless specified explicitly.

Signed-off-by: Raghavendra D Prabhu <rprabhu@wnohang.net>
Signed-off-by: Karel Zak <kzak@redhat.com>