message: add flow for the plugin api
diff: @@ -2,7 +2,14 @@ import Validator from './core/validator';
  import ErrorBag from './core/errorBag';
  import { warn, isCallable } from './core/utils';
  -function use (plugin, options = {}) {
  +// @flow
  +type PluginContext = {
  + Validator: Validator,
  + ErrorBag: ErrorBag,
  + Rules: Object,
  +};
  +
  +function use (plugin: (ctx: PluginContext, options?: any) => any, options?: any = {}) {
  if (!isCallable(plugin)) {
  return warn('The plugin must be a callable function');
  }
  