Helper to convert config values to boolean as ConfigParser do.

def _cast_boolean(value):
    """
    Helper to convert config values to boolean as ConfigParser do.
    """
    _BOOLEANS = {'1': True, 'yes': True, 'true': True, 'on': True,
                 '0': False, 'no': False, 'false': False, 'off': False, '': False}
    value = str(value)
    if value.lower() not in _BOOLEANS:
        raise ValueError('Not a boolean: %s' % value)

    return _BOOLEANS[value.lower()]