Implement kill command

[piotrminkina]

I think kill command with optional -s parameter (to specify signal type) will be helpful for the following case:

Example Procfile:

confd: exec confd -backend etcd -watch -config-file myconfig.toml
nginx: exec nginx -g 'daemon off;'

Example myconfig.toml:

[template]
src = "/etc/nginx/nginx.conf.tmpl"
dest = "/etc/nginx/nginx.conf"
reload_cmd = "spm kill -s HUP nginx"

Thanks for this command we can send some signals to running jobs, without known its current pid.

Is it possible that this feature will be implemented?