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

pkg/args/test/usage_test.dart

Index: pkg/args/test/usage_test.dart
diff --git a/pkg/args/test/usage_test.dart b/pkg/args/test/usage_test.dart
index 77d66a83153c630248e6104d56547f7cc6d5e09f..b6b860d4a2b0509968f569f6fff6b0a21721f1cb 100644
--- a/pkg/args/test/usage_test.dart
+++ b/pkg/args/test/usage_test.dart
@@ -122,6 +122,17 @@ void main() {
           ''');
     });
 
+    test('the value help is shown', () {
+      var parser = new ArgParser();
+      parser.addOption('out', abbr: 'o', help: 'Where to write file',
+          valueHelp: 'path');
+
+      validateUsage(parser,
+          '''
+          -o, --out=<path>    Where to write file
+          ''');
+    });
+
     test('the allowed list is shown', () {
       var parser = new ArgParser();
       parser.addOption('suit', help: 'Like in cards',