code: function validateOption (options, name, values) {
    if (options[name] !== undefined && !contains(values, options[name])) {
      const index = findIndex(values, options[name])
      if (index !== -1) {
        // right value, wrong casing
        // TODO: lower case values are deprecated since v3, remove this warning some day.
        console.warn('Warning: Wrong casing for configuration option "' + name + '", should be "' + values[index] + '" instead of "' + options[name] + '".')
  
        options[name] = values[index] // change the option to the right casing
      } else {
        // unknown value
        console.warn('Warning: Unknown value "' + options[name] + '" for configuration option "' + name + '". Available options: ' + values.map(JSON.stringify).join(', ') + '.')
      }
    }
  }
docs: Validate an option
  @param {Object} options         Object with options
  @param {string} name            Name of the option to validate
  @param {Array.<string>} values  Array with valid values for this option