Feature request: allow adding groups to groups

[chingologram]

It would be useful to allow [group] entries to be defined around groups as well as programs.
This would allow for easier management and control of processes.

[group:my-big-group]
groups=subgroup-1,subgroup-2

[group:subgroup-1]
programs=program-a,program-b

[group:subgroup-2]
programs=program-c,program-d

[group:another-group]
programs-program-z

Currently turning off all groups would require explicitly stopping the subgroups 1 and 2 with a series of supervisorctl stop subgroup-1:* supervisorctl stop subgroup-2:*.
sudo service supervisorctl stop here wouldn't work because we might want to leave another-group running.
With the above configuration, this could be resolved with supervisorctl stop my-big-group:*
There might be situations where there are not 2 but several groups of programs, and to manage all, a lot of commands would be required, which increases the chance for human error and the cost to maintain scripts.