registerOption (key, value, pluginName) {
    const option = PLUGIN_OPTION_MAP[key]
    const types = option.types
    const { valid, warnMsg } = assertTypes(value, types)
    if (valid) {
      this.options[option.name].add(pluginName, value)
    } else if (value !== undefined) {
      logger.warn(
        `${chalk.gray(pluginName)} `
        + `Invalid value for "option" ${chalk.cyan(option.name)}: ${warnMsg}`
      )
    }
    return this
  }
Register plugin option.

@param {string} key
@param {any} value
@param {string} pluginName
@returns {module.PluginAPI}
@api private