Add support for hidden options to pkg/args.

pkg/args/lib/src/options.dart

 library options;
 
 import 'package:unmodifiable_collection/unmodifiable_collection.dart';
 
 /**
  * A command-line option. Includes both flags and options which take a value.
  */
 class Option {
   final String name;
   final String abbreviation;
   final List<String> allowed;
   final defaultValue;
   final Function callback;
   final String help;
   final Map<String, String> allowedHelp;
   final bool isFlag;
   final bool negatable;
   final bool allowMultiple;
+  final bool isHidden;
 
   Option(this.name, this.abbreviation, this.help, List<String> allowed,
       Map<String, String> allowedHelp, this.defaultValue, this.callback,
-      {this.isFlag, this.negatable, this.allowMultiple: false}) :
+      {this.isFlag, this.negatable, this.allowMultiple: false,
+      this.isHidden: false}) :
         this.allowed = allowed == null ?
             null : new UnmodifiableListView(allowed),
         this.allowedHelp = allowedHelp == null ?
             null : new UnmodifiableMapView(allowedHelp) {
 
     if (name.isEmpty) {
       throw new ArgumentError('Name cannot be empty.');
     } else if (name.startsWith('-')) {
       throw new ArgumentError('Name $name cannot start with "-".');
     }
@@ skipping 11 matching lines @@
       }
 
       if (_invalidChars.hasMatch(abbreviation)) {
         throw new ArgumentError('Abbreviation is an invalid character.');
       }
     }
   }
 
   static final _invalidChars = new RegExp(r'''[ \t\r\n"'\\/]''');
 }