Dynamic Text Sizes for Text component

Summary:
Dynamic Text Sizes for Text component.
Text gains new prop - allowFontScaling (true by default).
There is also AccessibilityManager module that allows you to tune multipliers per each content size category, but predefined multipliers are there.
This could potentially break some apps so please test carefully.

Examples/UIExplorer/TextExample.ios.js

@@ -369,6 +369,25 @@ exports.examples = [
       </View>
     );
   },
+}, {
+  title: 'allowFontScaling attribute',
+  render: function() {
+    return (
+      <View>
+        <Text>
+          By default, text will respect Text Size accessibility setting on iOS. 
+          It means that all font sizes will be increased or descreased depending on the value of Text Size setting in
+          {" "}<Text style={{fontWeight: 'bold'}}>Settings.app - Display & Brightness - Text Size</Text>
+        </Text>
+        <Text style={{marginTop: 10}}>
+          You can disable scaling for your Text component by passing {"\""}allowFontScaling={"{"}false{"}\""} prop.
+        </Text>
+        <Text allowFontScaling={false} style={{marginTop: 20}}>
+          This text will not scale.
+        </Text>
+      </View>
+    );
+  },
 }];
 
 var styles = StyleSheet.create({