Possibility to overwrite label's style if defined as string. (#731)

* Added option to overwrite label style for only if label is as string defined in navigationOptions in DrawerNavigation.

* update doc for labelStyle prop

* change title to label

docs/api/navigators/DrawerNavigator.md

@@ -137,6 +137,7 @@ const styles = StyleSheet.create({
 - `inactiveTintColor` - label and icon color of the inactive label
 - `inactiveBackgroundColor` - background color of the inactive label
 - `style` - style object for the content section
+- `labelStyle` - style object to overwrite `Text` style inside content section, when your label is a string
 
 #### Example:
 

src/views/Drawer/DrawerNavigatorItems.js

@@ -29,6 +29,7 @@ type Props = {
   getLabel: (scene: DrawerScene) => ?(React.Element<*> | string);
   renderIcon: (scene: DrawerScene) => ?React.Element<*>;
   style?: Style;
+  labelStyle?: Style;
 };
 
 /**
@@ -43,6 +44,7 @@ const DrawerNavigatorItems = ({
   getLabel,
   renderIcon,
   style,
+  labelStyle,
 }: Props) => (
   <View style={[styles.container, style]}>
     {navigation.state.routes.map((route: *, index: number) => {
@@ -69,7 +71,7 @@ const DrawerNavigatorItems = ({
             ) : null}
             {typeof label === 'string'
               ? (
-                <Text style={[styles.label, { color }]}>
+                <Text style={[styles.label, { color }, labelStyle]}>
                   {label}
                 </Text>
               )