make mkdir options optional
make mkdir options optional

This PR simplifies the code slightly by using a Javascript default value for `mkdir`'s `options` parameter. It also fixes a flow error that otherwise occurs when `mkdir` is called with only one argument.
johanneslumpe/react-native-fs,itinance/react-native-fs,itinance/react-native-fs,itinance/react-native-fs,johanneslumpe/react-native-fs,itinance/react-native-fs,johanneslumpe/react-native-fs