Escape string interpolation character

```
$ cookiecutter -h
Traceback (most recent call last):
  File "/home/michael/.virtualenvs/cookiecutter/bin/cookiecutter", line 9, in <module>
    load_entry_point('cookiecutter==0.7.2', 'console_scripts', 'cookiecutter')()
  File "/home/michael/Source/cookiecutter/cookiecutter/main.py", line 145, in main
    args = parse_cookiecutter_args(sys.argv[1:])
  File "/home/michael/Source/cookiecutter/cookiecutter/main.py", line 135, in parse_cookiecutter_args
    arguments = _get_parser().parse_args(args)
  File "/usr/lib/python2.7/argparse.py", line 1690, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/lib/python2.7/argparse.py", line 1722, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib/python2.7/argparse.py", line 1928, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/usr/lib/python2.7/argparse.py", line 1868, in consume_optional
    take_action(action, args, option_string)
  File "/usr/lib/python2.7/argparse.py", line 1796, in take_action
    action(self, namespace, argument_values, option_string)
  File "/usr/lib/python2.7/argparse.py", line 996, in __call__
    parser.print_help()
  File "/usr/lib/python2.7/argparse.py", line 2329, in print_help
    self._print_message(self.format_help(), file)
  File "/usr/lib/python2.7/argparse.py", line 2303, in format_help
    return formatter.format_help()
  File "/usr/lib/python2.7/argparse.py", line 281, in format_help
    help = self._root_section.format_help()
  File "/usr/lib/python2.7/argparse.py", line 211, in format_help
    func(*args)
  File "/usr/lib/python2.7/argparse.py", line 211, in format_help
    func(*args)
  File "/usr/lib/python2.7/argparse.py", line 517, in _format_action
    help_text = self._expand_help(action)
  File "/usr/lib/python2.7/argparse.py", line 603, in _expand_help
    return self._get_help_string(action) % params
ValueError: unsupported format character 'Y' (0x59) at index 92
```