add parse args

prettyjson.dart

@@ -1,8 +1,20 @@
 import 'dart:convert';
 import 'dart:io';
+import 'package:args/args.dart';
 
 main(List<String> args) async {
-  final jsonContent = await readContent(args);
+  final parser = new ArgParser();
+  parser.addFlag('help', abbr: 'h', defaultsTo: false, help: 'Help');
+
+  final parseResult = parser.parse(args);
+  if (parseResult['help']) {
+    print('Print help message for prettyjson.dart');
+    return;
+  }
+
+  final restArgs = parseResult.rest;
+
+  final jsonContent = await readContent(restArgs);
   final jsonDecoded = json.decode(jsonContent);
   JsonEncoder encoder = new JsonEncoder.withIndent('    ');
   print(encoder.convert(jsonDecoded));

pubspec.yaml

@@ -1,3 +1,5 @@
 name:
   prettyjson.dart
 
+dependencies:
+  args: ^1.5.2