Check Sass style

* Use open source SCSS-Lint https://github.com/causes/scss-lint
* Based on https://github.com/thoughtbot/hound/pull/406
* Use SCSS config from Bourbon Neat
  https://github.com/thoughtbot/neat/blob/master/.scss-lint.yml
This should be `DEFAULT_CONFIG_FILENAME` and just provide the filename. We should use `DefaultConfigFile` to grab either the thoughtbot config, or the default one.