function _updateFormatOut (formatOut, options) {
  if (!(is.object(options) && options.force === false)) {
    this.options.formatOut = formatOut;
  }
  return this;
}
Update the output format unless options.force is false,
in which case revert to input format.
@private
@param {string} formatOut
@param {Object} [options]
@param {boolean} [options.force=true] - force output format, otherwise attempt to use input format
@returns {Sharp}