Actually run the processes for the start command, and watch for error and
exit events for each of the proc items.

Add KillProcs function to kill all the running processes.

Add fields to Proc to manage the running processes and channels to send
error and exit events.

Add Kill method to *Proc that'll kill all the still running processes.

Add Start method to *Proc, that runs all the processes setting the PORT
variable in the environment and replacing any PORT variables in the
arguments.