title: Border bottom on Bottom Navigation in React Native
body: <p>I am new to React Native. I am making a bottom navigation bar. I want to add  border bottom when the tab is selected, like the picture below. According to the docs, the <em>indicatorStyle</em> is only applicable on Top Navigation. I am unsure on how to do it.</p>
  
  <pre><code>import {
    createBottomTabNavigator,
  } from 'react-navigation';
  import SettingsScreen from '../settings';
  import rootStyles from '../../rootStyles';
  const ICON_HOME = require('../../../assets/icon/home.png');
  const ICON_USER = require('../../../assets/icon/user_selected.png');
  const ICON_CURRENCY = require('../../../assets/icon/currency.png');
  
  
  export default createBottomTabNavigator({
    HomeScreen: {
      screen: SettingsScreen,
    },
    SettingsScreen: {
      screen: SettingsScreen,
    },
    CurrencyScreen: {
      screen: SettingsScreen,
    },
  }, {
    navigationOptions: ({ navigation }) =&gt; ({
      tabBarIcon: ({ tintColor }) =&gt; {
        const { routeName } = navigation.state;
        let icon;
  
        switch (routeName) {
          case 'HomeScreen':
            icon = ICON_HOME;
            break;
          case 'SettingsScreen':
            icon = ICON_USER;
            break;
          case 'CurrencyScreen':
            icon = ICON_CURRENCY;
            break;
          default: break;
        }
  
        return (
          &lt;Image
            source={icon}
            style={{
              height: rootStyles.em(2),
              width: rootStyles.em(2),
              tintColor,
            }}
          /&gt;
        );
      },
    }),
    tabBarPosition: 'bottom',
    tabBarOptions: {
      showLabel: false,
      activeTintColor: rootStyles.PRIMARY_COLOR,
    },
    tabBarSelectedItemStyle: {
      borderBottomWidth: 2,
      borderBottomColor: 'red',
    },
  });
  </code></pre>