Allow defining a help string for an option's parameter value.

pkg/args/lib/src/options.dart

 library options;
 
 import 'package:collection/wrappers.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 String valueHelp;
   final Map<String, String> allowedHelp;
   final bool isFlag;
   final bool negatable;
   final bool allowMultiple;
   final bool hide;
 
-  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,
+  Option(this.name, this.abbreviation, this.help, this.valueHelp,
+      List<String> allowed, Map<String, String> allowedHelp, this.defaultValue,
+      this.callback, {this.isFlag, this.negatable, this.allowMultiple: false,
       this.hide: 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 12 matching lines @@
       }
 
       if (_invalidChars.hasMatch(abbreviation)) {
         throw new ArgumentError('Abbreviation is an invalid character.');
       }
     }
   }
 
   static final _invalidChars = new RegExp(r'''[ \t\r\n"'\\/]''');
 }