Add a config to override the truncated back title (#1137)

Fixes #1136.

docs/api/navigators/StackNavigator.md

@@ -107,6 +107,10 @@ String or React Element used by the header. Defaults to scene `title`
 
 Title string used by the back button on iOS or `null` to disable label. Defaults to scene `title`
 
+#### `headerTruncatedBackTitle`
+
+Title string used by the back button when `headerBackTitle` doesn't fit on the screen. `"Back"` by default.
+
 #### `headerRight`
 
 String or React Element to display on the right side of the header

src/TypeDefinition.js

@@ -201,6 +201,7 @@ export type NavigationStackScreenOptions = NavigationScreenOptions & {
   headerTintColor?: string,
   headerLeft?: React.Element<*>,
   headerBackTitle?: string,
+  headerTruncatedBackTitle?: string,
   headerPressColorAndroid?: string,
   headerRight?: React.Element<*>,
   headerStyle?: Style,

src/views/Header.js

@@ -123,6 +123,7 @@ class Header extends React.PureComponent<void, HeaderProps, HeaderState> {
         pressColorAndroid={options.headerPressColorAndroid}
         tintColor={options.headerTintColor}
         title={backButtonTitle}
+        truncatedTitle={options.headerTruncatedBackTitle}
         width={width}
       />
     );