'new_group' option

By default, jobs in a file are part of the same reporting group
unless seperated by a stone wall or if they define a group by
themselves (numjobs=x is used). Inserting a 'new_group' parameter
in a job description, will mark the start of a new reporting group.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>