Fix iOS Picker Item Colors

Summary:
I want to resolve #11170 by passing the `color` prop from `PickerIOS.Item` to its implementation.

In `RCTPicker.m`, the label.textColor was already being set and used, but there was nothing referencing the past prop. I passed the prop to the implementation, checked if it exists, and if not, set the default color, like before.

I visually tested the **Colorful Pickers** example in UIExplorer. Those picker `Item`s pass in a `color` prop.

![dec-01-2016 22-07-46](https://cloud.githubusercontent.com/assets/9259509/20821696/ae45d704-b812-11e6-9720-0045d6c0bcd4.gif)

The basic picker does not pass the color prop to the picker `Item`, and there are no errors. Basic functionality is still in tact:

![dec-01-2016 22-09-35](https://cloud.githubusercontent.com/assets/9259509/20821730/ee544f74-b812-11e6-9294-a1b45e78d9f7.gif)
Closes #11260

Differential Revision: D4272370

fbshipit-source-id: 5fa33c40526dda59ca2ab527c31351bcd27e5cf3

Author: JakeDawkins

Libraries/Components/Picker/PickerIOS.ios.js

@@ -18,6 +18,7 @@ var StyleSheet = require('StyleSheet');
 var StyleSheetPropType = require('StyleSheetPropType');
 var TextStylePropTypes = require('TextStylePropTypes');
 var View = require('View');
+var processColor = require('processColor');
 
 var itemStylePropType = StyleSheetPropType(TextStylePropTypes);
 var requireNativeComponent = require('requireNativeComponent');
@@ -48,7 +49,11 @@ var PickerIOS = React.createClass({
       if (child.props.value === props.selectedValue) {
         selectedIndex = index;
       }
-      items.push({value: child.props.value, label: child.props.label});
+      items.push({
+        value: child.props.value,
+        label: child.props.label,
+        textColor: processColor(child.props.color),
+      });
     });
     return {selectedIndex, items};
   },
@@ -95,6 +100,7 @@ PickerIOS.Item = class extends React.Component {
   static propTypes = {
     value: React.PropTypes.any, // string or integer basically
     label: React.PropTypes.string,
+    color: React.PropTypes.string,
   };
 
   render() {

React/Views/RCTPicker.m

@@ -86,7 +86,9 @@ - (UIView *)pickerView:(UIPickerView *)pickerView
   }
 
   label.font = _font;
-  label.textColor = _color;
+
+  label.textColor = [RCTConvert UIColor:_items[row][@"textColor"]] ?: _color;
+
   label.textAlignment = _textAlign;
   label.text = [self pickerView:pickerView titleForRow:row forComponent:component];
   return label;