Extend RNPicker props

[hamishhossack]

So we can use itemStyle and other options without syntax errors.

<WheelPicker
    selectedItem={this.state.selectedItem}
    data={wheelPickerData}
    onItemSelected={this.onItemSelected}
    // Add itemStyle as per @react-native-community/picker docs
    // @reference https://github.com/react-native-picker/picker#props-1
    itemStyle={{ color: "white" }}
    {...moreIosProps}
  />

It would also remove both of these users "syntax errors" since it does actually work if you pass those props.
#160
#159