add options to set default Locale

[juncaixinchi]

Example to set default locale

import 'package:flutter/material.dart';
import 'package:intl/intl_standalone.dart';

import 'package:flutter_i18n/flutter_i18n_delegate.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

void main() async {
  // set default language or load from configs
  String lan = 'en';
  if (lan == null) {
    // load system locale if lan == null
    final String systemLocale = await findSystemLocale();
    final List<String> systemLocaleSplitted = systemLocale.split('_');
    lan = systemLocaleSplitted[0];
  }

  final FlutterI18nDelegate flutterI18nDelegate = FlutterI18nDelegate(
    useCountryCode: false,
    fallbackFile: 'en',
    path: 'assets/locales',
    defaultLocale: Locale(lan),
  );

  // preload default locale to avoid black screen
  await flutterI18nDelegate.load(null);

  runApp(MyApp(flutterI18nDelegate));
}

class MyApp extends StatelessWidget {
  final FlutterI18nDelegate flutterI18nDelegate;

  MyApp(this.flutterI18nDelegate);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Text('Test'),
        ),
      ),
      localizationsDelegates: [
        flutterI18nDelegate,
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate
      ],
    );
  }
}