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

Fixes #1136.

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}
       />
     );