Added option privileged_mode which defaults to false

This PR adds the option :privileged_mode to allow running docker containers with -privileged=true. This was necessary in our case to permit mounting volumes inside the container, something we are doing in some cookbooks. 

Example .kitchen.yml with this option:

driver_plugin: docker
provisioner: chef_zero
platforms:
- name: centos-6.4
  driver_config:
    require_chef_omnibus: 11.6.0
    privileged_mode: true
suites:
- name: default
  run_list:
    - recipe[selenium]

README was updated with some info about this option.