add rlimit configuration for programs

Hi, I've used a hacked up supervisord for years with the options:
```ini
[program:prog1]
command=/bin/cat
minprocs=4096
minfds=4096
stacksize=10240
```
Which directs supervisor to call setrlimit to the values just after forking to launch the program. For RLIMITS with a soft and hard limit, I've just forced the 'min' value to both since most programs under supervisord are not equipped to call setrlimit on their own behalf.

I think this is generally useful, and I'd love to stop having to manually patch to get updates. Let me know if I've abused any code or if any code style is not good.