Add iOS 10 textContentType for TextInput

Summary:
Setting `textContentType` will provide the keyboard and system with semantic meaning for inputs. Should enable password/username autofill in apps running on iOS 11+ as demonstrated here: https://developer.apple.com/videos/play/wwdc2017/206/

Also gives you the ability to disable autofill by setting `textContentType="none"`: https://stackoverflow.com/questions/48489479/react-native-disable-password-autofill-option-on-ios-keyboard

Adding `textContentType` equal to `username` or `password`  should give you an autofill-bar over the keyboard which will let you fill in values from the device Keychain:

![image](https://user-images.githubusercontent.com/4932625/37848513-b2170490-2ed4-11e8-85bf-895823d4f98a.png)

Setting the appropriate `textContentType` will fill in the correct value in the `TextInput`.
I have only been able to get this to work on device, and not simulator.

Usage:
```jsx
<TextInput
    value={this.state.username}
    onChangeText={this.setUserName}
    textContentType="username"
/>
```

```jsx
<TextInput
    value={this.state.password}
    onChangeText={this.setPassword}
    secureTextEntry={true}
    textContentType="password"
/>
```

To disable:
```jsx
<TextInput
    value={this.state.password}
    onChangeText={this.setPassword}
    secureTextEntry={true}
    textContentType="none"
/>
```

This will set `textContentType` to an empty string: https://stackoverflow.com/a/46474180/5703116

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Docs PR coming up.

[IOS] [MINOR] [TextInput] - Added `textContentType` prop for iOS 10+. Will enable password autofill for iOS 11+.
Closes https://github.com/facebook/react-native/pull/18526

Differential Revision: D7469630

Pulled By: hramos

fbshipit-source-id: 852a9749be98d477ecd82154c0a65a7c084521c1

Libraries/Text/TextInput/RCTBaseTextInputView.m

@@ -167,6 +167,17 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithFrame:(CGRect)frame)
   }
 }
 
+- (void)setTextContentType:(NSString *)type
+{
+  #if defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_10_0
+    if (@available(iOS 10.0, *)) {
+        // Setting textContentType to an empty string will disable any
+        // default behaviour, like the autofill bar for password inputs
+        self.backedTextInputView.textContentType = [type isEqualToString:@"none"] ? @"" : type;
+    }
+  #endif
+}
+
 #pragma mark - RCTBackedTextInputDelegate
 
 - (BOOL)textInputShouldBeginEditing