Work on debouncer.dart

- improved API and docs.

 import 'dart:async';
 
+/// This "function" class is the implementation of [debouncer()] Worker.
+/// It calls the function passed after specified [delay] parameter.
+/// Example:
+/// ```
+/// final delayed = Debouncer( delay: Duration( seconds: 1 )) ;
+/// print( 'the next function will be called after 1 sec' );
+/// delayed( () => print( 'called after 1 sec' ));
+/// ```
 class Debouncer {
   final Duration delay;
   Timer _timer;
 
   Debouncer({this.delay});
 
-  call(void Function() action) {
+  void call(void Function() action) {
     _timer?.cancel();
     _timer = Timer(delay, action);
   }
+
+  /// Notifies if the delayed call is active.
+  bool get isRunning => _timer?.isActive ?? false;
+
+  /// Cancel the current delayed call.
+  void cancel() => _timer?.cancel();
 }