docs: def tag():
      """Tag current version."""
      if check_unstaged():
          raise EnvironmentError('There are staged changes, abort.')
      with open(str(INIT_PATH)) as f:
          metadata = dict(re.findall("__([a-z]+)__ = '([^']+)'", f.read()))
      version = metadata['version']
      check_output(['git', 'tag', version, '-m', 'Release v{}'.format(version)])