Allow TabBarBottom accept onTabPress callback (#486)

Pressing a tab bar often requires performing some kind of side effects
other than navigating: _Scroll-to-top_ functionality, for instance.
At the moment `TabView.TabBarTop`, which is the default on _Android_,
handles `onTabPress` callback as expected, but `TabView.TabBarBottom`
ignores it.

This PR addresses the problem so that `TabView.TabBarBottom` can accept
and handle the optional `onTabPress` callback. The callback is invoked
exactly once if present on every tab bar press, accepting as a sole
parameter a `NavigationRoute`, which is the route object the pressed
target represents.

docs/api/navigators/TabNavigator.md

@@ -108,6 +108,7 @@ Several options get passed to the underlying router to modify navigation logic:
 - `showLabel` - whether to show label for tab, default is true
 - `style` - style object for the tab bar
 - `labelStyle` - style object for the tab label
+- `onTabPress` - optional callback invoked on tab press
 
 Example:
 
@@ -135,6 +136,7 @@ tabBarOptions: {
 - `indicatorStyle` - style object for the tab indicator (line at the bottom of the tab)
 - `labelStyle` - style object for the tab label
 - `style` - style object for the tab bar
+- `onTabPress` - optional callback invoked on tab press
 
 Example:
 

src/views/TabView/TabBarBottom.js

@@ -35,6 +35,7 @@ type Props = {
   position: Animated.Value;
   navigationState: NavigationState;
   jumpToIndex: (index: number) => void;
+  onTabPress: (route: NavigationRoute) => void;
   getLabel: (scene: TabScene) => ?(React.Element<*> | string);
   renderIcon: (scene: TabScene) => React.Element<*>;
   showLabel: boolean;
@@ -57,6 +58,18 @@ export default class TabBarBottom extends PureComponent<DefaultProps, Props, voi
 
   props: Props;
 
+  _handleTabPress = (index: number) => {
+    const {
+      navigationState,
+      jumpToIndex,
+      onTabPress,
+    } = this.props;
+    jumpToIndex(index);
+    if (onTabPress) {
+      onTabPress(navigationState.routes[index]);
+    }
+  }
+
   _renderLabel = (scene: TabScene) => {
     const {
       position,
@@ -144,7 +157,10 @@ export default class TabBarBottom extends PureComponent<DefaultProps, Props, voi
           });
           const justifyContent = this.props.showIcon ? 'flex-end' : 'center';
           return (
-            <TouchableWithoutFeedback key={route.key} onPress={() => jumpToIndex(index)}>
+            <TouchableWithoutFeedback
+              key={route.key}
+              onPress={() => this._handleTabPress(index)}
+            >
               <Animated.View style={[styles.tab, { backgroundColor, justifyContent }]}>
                 {this._renderIcon(scene)}
                 {this._renderLabel(scene)}