Document that Command.argParser can be overridden. (#72)

See #68
diff --git a/lib/command_runner.dart b/lib/command_runner.dart
index 2fe1517..13b0a6e 100644
--- a/lib/command_runner.dart
+++ b/lib/command_runner.dart
@@ -267,6 +267,9 @@
   /// the constructor); they'll end up available via [argResults]. Subcommands
   /// should be registered with [addSubcommand] rather than directly on the
   /// parser.
+  ///
+  /// This can be overridden to change the arguments passed to the `ArgParser`
+  /// constructor.
   ArgParser get argParser => _argParser;
   final _argParser = new ArgParser();