feat(NavbarToggler): add aria-label to NavbarToggler

This adds the aria-label to the NavbarToggler as
seen in the bootstrap docs.

Fixes #1632

src/NavbarToggler.js

@@ -31,7 +31,7 @@ const NavbarToggler = (props) => {
   ), cssModule);
 
   return (
-    <Tag {...attributes} className={classes}>
+    <Tag {...attributes} className={classes} aria-label="Toggle navigation">
       {children || <span className={mapToCssModules('navbar-toggler-icon', cssModule)} />}
     </Tag>
   );

src/__tests__/NavbarToggler.spec.js

@@ -5,7 +5,8 @@ import { NavbarToggler } from '../';
 describe('NavbarToggler', () => {
   it('should render .navbar-toggler markup', () => {
     const wrapper = shallow(<NavbarToggler />);
-
+
+    expect(wrapper.prop('aria-label')).toBe('Toggle navigation');
     expect(wrapper.html()).toBe('<button type="button" class="navbar-toggler"><span class="navbar-toggler-icon"></span></button>');
   });