message: Expose the --skip-selenium-install option.
  (Towards polymer/web-component-tester#215)
  This behavior for this option was already built-in to the plugin but
  there was no way to enable it.
diff: "seleniumPort": {
  "help": "The port of a running selenium server to use.",
  "full": "selenium-port"
  + },
  + "skipSeleniumInstall": {
  + "help": "Skip trying to install selenium.",
  + "full": "skip-selenium-install",
  + "flag": true
  }
  }
  },
  