injectDefaultFields = (config, path, type, front_matter = {}) => {
  let defaults;
  try {
    defaults = config.content.defaults;
  } catch (e) {
    return {};
  }
  let metafields = {};
  _.each(defaults, field => {
    const scope = field.scope;
    if (
      (!scope.type || scope.type === type) &&
      (!scope.path || scope.path === path)
    ) {
      _.extend(metafields, field.values);
    }
  });
  return _.extend(metafields, front_matter);
}
Injects the relevant default front matter fields read from the config file into
the provided front matter object
@param {Object} config
@param {String} path
@param {String} type
@param {Object} front_matter
@return {Object} metadata