Add keyboardAppearance prop to TextInput component.

Summary: Resolves #3649.
Closes https://github.com/facebook/react-native/pull/4012

Reviewed By: javache

Differential Revision: D2636538

Pulled By: nicklockwood

fb-gh-sync-id: 022e79d8f8fa684cad43af1a51f728d60ac652a8

React/Base/RCTConvert.h

@@ -63,6 +63,7 @@ typedef NSURL RCTFileURL;
 + (UITextAutocapitalizationType)UITextAutocapitalizationType:(id)json;
 + (UITextFieldViewMode)UITextFieldViewMode:(id)json;
 + (UIKeyboardType)UIKeyboardType:(id)json;
++ (UIKeyboardAppearance)UIKeyboardAppearance:(id)json;
 + (UIReturnKeyType)UIReturnKeyType:(id)json;
 
 + (UIViewContentMode)UIViewContentMode:(id)json;

React/Base/RCTConvert.m

@@ -272,6 +272,12 @@ RCT_ENUM_CONVERTER(UIKeyboardType, (@{
   @"numeric": @(UIKeyboardTypeDecimalPad),
 }), UIKeyboardTypeDefault, integerValue)
 
+RCT_ENUM_CONVERTER(UIKeyboardAppearance, (@{
+  @"default": @(UIKeyboardAppearanceDefault),
+  @"light": @(UIKeyboardAppearanceLight),
+  @"dark": @(UIKeyboardAppearanceDark),
+}), UIKeyboardAppearanceDefault, integerValue)
+
 RCT_ENUM_CONVERTER(UIReturnKeyType, (@{
   @"default": @(UIReturnKeyDefault),
   @"go": @(UIReturnKeyGo),