add support for additional parameters to be passed to the docker run cmd.

I was basically trying to run a additional docker package that starts the ports on randomly exposed container ports. I want to start the run command and tell it which ports to assign so I can have another docker container connect to them in a predictable manner.

This is the expected usage inside the vagrant file
          logstash: { image: 'ehazlett/logstash', additional_run_args: '-p 49176:5601 -p 49175:514', cmd: '/usr/local/bin/run'}

this will allow users to pass arbitrary arguments to the docker run command. Another option would be to support a more specific arg like port: but since it can be passed multiple times and there are other options people might want to use I thought that a generic arg might make more sense.