Add support for separators.

lib/src/arg_parser.dart

Index: lib/src/arg_parser.dart
diff --git a/lib/src/arg_parser.dart b/lib/src/arg_parser.dart
index 49d6aa6bbf6dc4d7a965536049fd8c305e87006e..c32741df94e4115e82e8dedfa3ee75088f88aeb8 100644
--- a/lib/src/arg_parser.dart
+++ b/lib/src/arg_parser.dart
@@ -23,6 +23,10 @@ class ArgParser {
   /// The commands that have been defined for this parser.
   final Map<String, ArgParser> commands;
 
+  /// A list of the [Option]s in [options] intermingled with [String]
+  /// separators.
+  final _optionsAndSeparators = [];
+
   /// Whether or not this parser parses options that appear after non-option
   /// arguments.
   final bool allowTrailingOptions;
@@ -110,9 +114,19 @@ class ArgParser {
       }
     }
 
-    _options[name] = newOption(name, abbr, help, valueHelp, allowed,
+    var option = newOption(name, abbr, help, valueHelp, allowed,
         allowedHelp, defaultsTo, callback, type,
         negatable: negatable, splitCommas: splitCommas, hide: hide);
+    _options[name] = option;
+    _optionsAndSeparators.add(option);
+  }
+
+  /// Adds a separator line to the usage.
+  ///
+  /// In the usage text for the parser, this will appear between any options
+  /// added before this call and ones added after it.
+  void addSeparator(String text) {
+    _optionsAndSeparators.add(text);
   }
 
   /// Parses [args], a list of command-line arguments, matches them against the
@@ -124,12 +138,12 @@ class ArgParser {
   ///
   /// This is basically the help text shown on the command line.
   @Deprecated("Replaced with get usage. getUsage() will be removed in args 1.0")
-  String getUsage() => new Usage(this).generate();
+  String getUsage() => usage;
 
   /// Generates a string displaying usage information for the defined options.
   ///
   /// This is basically the help text shown on the command line.
-  String get usage => new Usage(this).generate();
+  String get usage => new Usage(_optionsAndSeparators).generate();
 
   /// Get the default value for an option. Useful after parsing to test if the
   /// user specified something other than the default.