add headerBackTitleStyle screen option (#1083)

* add headerBackTitleStyle screen option

since the title is customizable, the back title should too.

* add documentation for `headerBackTitleStyle`

* make absence of headerBackTitle not affect return value

* merge styles so current scene can override

If there's no last scene, return the `headerBackTitleStyle` from the
current scene.
Otherwise return the `headerTitleStyle` from the last scene, and the
`headerBackTitleStyle` from the current scene, with the latter having
precedence.

* take headerBackTitleStyle from current scene

also the title style of the last scene is no longer taken into account

docs/api/navigators/StackNavigator.md

@@ -128,6 +128,10 @@ Style object for the header
 
 Style object for the title component
 
+#### `headerBackTitleStyle`
+
+Style object for the back title
+
 #### `headerTintColor`
 
 Tint color for the header

src/TypeDefinition.js

@@ -221,6 +221,7 @@ export type NavigationStackScreenOptions = NavigationScreenOptions & {
   headerLeft?: React.Element<*>,
   headerBackTitle?: string,
   headerTruncatedBackTitle?: string,
+  headerBackTitleStyle?: Style,
   headerPressColorAndroid?: string,
   headerRight?: React.Element<*>,
   headerStyle?: Style,

src/views/Header.js

@@ -136,6 +136,7 @@ class Header extends React.PureComponent<void, HeaderProps, HeaderState> {
         tintColor={options.headerTintColor}
         title={backButtonTitle}
         truncatedTitle={truncatedBackButtonTitle}
+        titleStyle={options.headerBackTitleStyle}
         width={width}
       />
     );

src/views/HeaderBackButton.js

@@ -10,7 +10,7 @@ import {
   StyleSheet,
 } from 'react-native';
 
-import type { LayoutEvent } from '../TypeDefinition';
+import type { LayoutEvent, Style } from '../TypeDefinition';
 
 import TouchableItem from './TouchableItem';
 
@@ -18,6 +18,7 @@ type Props = {
   onPress?: () => void,
   pressColorAndroid?: ?string,
   title?: ?string,
+  titleStyle?: ?Style,
   tintColor?: ?string,
   truncatedTitle?: ?string,
   width?: ?number,
@@ -59,6 +60,7 @@ class HeaderBackButton extends React.PureComponent<DefaultProps, Props, State> {
       pressColorAndroid,
       width,
       title,
+      titleStyle,
       tintColor,
       truncatedTitle,
     } = this.props;
@@ -92,7 +94,7 @@ class HeaderBackButton extends React.PureComponent<DefaultProps, Props, State> {
             title &&
             <Text
               onLayout={this._onTextLayout}
-              style={[styles.title, { color: tintColor }]}
+              style={[styles.title, { color: tintColor }, titleStyle]}
               numberOfLines={1}
             >
               {backButtonTitle}