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

pkg/args/lib/src/usage.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library args.src.usage;
 
 import 'dart:math';
 
 import '../args.dart';
 
@@ skipping 90 matching lines @@
 
   String getAbbreviation(Option option) {
     if (option.abbreviation != null) {
       return '-${option.abbreviation}, ';
     } else {
       return '';
     }
   }
 
   String getLongOption(Option option) {
+    var result;
     if (option.negatable) {
-      return '--[no-]${option.name}';
+      result = '--[no-]${option.name}';
     } else {
-      return '--${option.name}';
+      result = '--${option.name}';
     }
+
+    if (option.valueHelp != null) result += "=<${option.valueHelp}>";
+
+    return result;
   }
 
   String getAllowedTitle(String allowed) {
     return '      [$allowed]';
   }
 
   void calculateColumnWidths() {
     int abbr = 0;
     int title = 0;
     args.options.forEach((name, option) {
@@ skipping 103 matching lines @@
 String padRight(String source, int length) {
   final result = new StringBuffer();
   result.write(source);
 
   while (result.length < length) {
     result.write(' ');
   }
 
   return result.toString();
 }