Support additional groups (--group-add)

[haines]

This PR adds an additional_groups option to the plugin which takes an array of groups (either names or ids), and translates them to --group-add options to pass to docker run.

I wanted to be able to do this because I'm trying to build Docker images as an unprivileged user, so I need to be in the docker group. My config would then be something like

docker#v1.1.1:
  image: "docker:latest"

  mounts:
    - /var/run/docker.sock:/var/run/docker.sock
    - # other mounts

  user: "501:502" # uid:gid of the user on the host, for access to the other mounts
  additional_groups:
    - 501 # gid of docker group on the host, for access to the docker socket

[toolmantim]

Thanks for th contribution! The PR looks great.