title: Add Gradient to Label of BottomNavigationBarItem
body: <p>In my flutter app I have a bottom navigation bar.
  I want to add a LinearGradient to the selected item.</p>
  <p>I managed to add a Gradient to the Icon by using the <code>activeIcon</code> attribute of BottomNavigationBarItem.</p>
  <p>However I don't know how to add the same gradient to the label of the item.
  Since <code>BottomNavigationBarItem.title</code> is deprecated and <code>BottomNavigationBarItem.label</code> only allows me to add a String I can not add the Gradient to the label directly.</p>
  <p>I have tried messing with the <code>selectedLabelStyle</code> of the BottomNavigationBar but had no success.</p>
  <p>Here is a screenshot of how my BottomNavigationBar looks.
  <a href="https://i.stack.imgur.com/azeVA.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/azeVA.jpg" alt="BottomNavigationBar" /></a>
  Currently the Label has a single color. I want it to have the same gradient as the icon.</p>
  <p>Does anyone know how I can achieve this?</p>
  <p>Thank you very much!</p>
  <pre><code>       BottomNavigationBar(
            onTap: onTabTapped,
            currentIndex: _currentPage,
            backgroundColor: context.appTheme.primaryBackgroundColor,
            type: BottomNavigationBarType.fixed,
            unselectedItemColor: context.appTheme.primaryTextColor,
            showUnselectedLabels: true,
            items: [
              BottomNavigationBarItem(
                activeIcon: GradientMask(
                  child: Icon(Icons.home_filled),
                  gradient: context.appTheme.primaryGradient,
                ),
                icon: Icon(Icons.home_filled),
                label: 'Home',
              ),
              BottomNavigationBarItem(
                activeIcon: GradientMask(
                  child: Icon(Icons.search),
                  gradient: context.appTheme.primaryGradient,
                ),
                icon: Icon(Icons.search),
                label: 'Search',
              ),
              BottomNavigationBarItem(
                activeIcon: GradientMask(
                  child: Icon(Icons.savings),
                  gradient: context.appTheme.primaryGradient,
                ),
                icon: Icon(Icons.savings),
                label: 'Earnings'
              ),
              BottomNavigationBarItem(
                activeIcon: GradientMask(
                  child: Icon(Icons.volunteer_activism),
                  gradient: context.appTheme.primaryGradient,
                ),
                icon: Icon(Icons.volunteer_activism),
                label: 'Sharing'
              ),
              BottomNavigationBarItem(
                activeIcon: GradientMask(
                  child: Icon(Icons.inventory_2),
                  gradient: context.appTheme.primaryGradient,
                ),
                icon: Icon(Icons.inventory_2),
                label: 'My Items'
              )
            ],
          ),
  </code></pre>