feat(UncontrolledDropdown): add onToggle callback (#1705)

Add `onToggle` to `UncontrolledDropdown`. The purpose of this new prop is to allow the user to be informed when the dropdown is toggled (opened/closed) as well as the new state of `isOpen` while still being uncontrolled.
when the dropdown is toggled, `onToggle` is called with the `event` and a `boolean` representing the new state (`true` mean the dropdown is being opened, `false` means it is being closed).

src/UncontrolledDropdown.js

@@ -13,8 +13,11 @@ export default class UncontrolledDropdown extends Component {
     this.toggle = this.toggle.bind(this);
   }
 
-  toggle() {
+  toggle(e) {
     this.setState({ isOpen: !this.state.isOpen });
+    if (this.props.onToggle) {
+      this.props.onToggle(e, !this.state.isOpen);
+    }
   }
 
   render() {
@@ -24,5 +27,6 @@ export default class UncontrolledDropdown extends Component {
 
 UncontrolledDropdown.propTypes = {
   defaultOpen: PropTypes.bool,
+  onToggle: PropTypes.func,
   ...Dropdown.propTypes
 };