Tool to interact with harpoon supervisor from the command line.

This is a simple tool for interacting with the harpoon supervisor from the command line.  I wrote it
because I needed to see what the supervisor was really sending back to the argent.  I think it will
generally be useful for debugging purposes.

I'm currently calling it containerctl, but that may not be the best name.  Other names I've contemplated
are harpoon-supervisorctl and supervisorctl.  Please let me know if there is one you prefer.

It might also make sense to eventually roll it into harpoonctl, but I'd want to wait until we see its actual
utility before doing that.

@bernerdschaefer @peterbourgon