Add style and imageStyle props to Navigation.Header.BackButton

Summary:
This PR adds`style` and `imageStyle` props to `Navigation.Header.BackButton` which allows for usage like:

```javascript
<NavigationExperimental.Header.BackButton
  imageStyle={{ tintColor: 'white' }}
  onPress={props.onNavigateBack}
/>
```
Closes #11554

Differential Revision: D4494460

Pulled By: mkonicek

fbshipit-source-id: 387a69d0b76bb8e7ef47caf1f6141f97fad0ea4b

Libraries/CustomComponents/NavigationExperimental/NavigationHeaderBackButton.js

@@ -35,12 +35,14 @@ const {
 } = ReactNative;
 
 type Props = {
+  imageStyle?: any,
   onPress: Function,
+  style?: any,
 };
 
 const NavigationHeaderBackButton = (props: Props) => (
-  <TouchableOpacity style={styles.buttonContainer} onPress={props.onPress}>
-    <Image style={styles.button} source={require('./assets/back-icon.png')} />
+  <TouchableOpacity style={[styles.buttonContainer, props.style]} onPress={props.onPress}>
+    <Image style={[styles.button, props.imageStyle]} source={require('./assets/back-icon.png')} />
   </TouchableOpacity>
 );