If no paths are provided, read source from stdin. Fix #165.

example/format.dart

+// Copyright (c) 2015, 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.
+
 import 'package:dart_style/dart_style.dart';
 
 import 'package:dart_style/src/debug.dart';
 
 void main(List<String> args) {
   // Enable debugging so you can see some of the formatter's internal state.
   // Normal users do not do this.
   debugFormatter = true;
   useAnsiColors = true;
 
@@ skipping 31 matching lines @@
     print(result);
   } on FormatterException catch (error) {
     print(error.message());
   }
 }
 
 void drawRuler(String label, int width) {
   var padding = " " * (width - label.length - 1);
   print("$label:$padding|");
 }