Add getopt to enable some simple configuration

And a usage message:

    usage: playpen [options] [command ...]
    Options:
     -h, --help                  display this help
     -v, --version               display version
     -u, --user=USER             the user to run the program as
     -r, --root=ROOT             the root of the container
     -n, --hostname=NAME         the hostname to set the container to
         --memory-limit=LIMIT    the memory limit of the container