added stop/cancel methods

lib/nice-tween.js

@@ -100,6 +100,29 @@ export default class NiceTween {
     return this;
   }
 
+  /**
+   * Stops the currently running tween and advances to the end of the tween so
+   * the target game object will be in whatever state is expected at the end of
+   * this tween.
+   */
+  stop ( ) {
+    this._progress = 1.0;
+    const term = this._easing(this._progress);
+
+    const current = { };
+    for (const property of this._properties) {
+      current[property] = NiceTween.lerp(this._from[property], this._to[property], term);
+    }
+
+    this.applyTargetUpdate(current);
+    this._state = 'finished';
+  }
+
+  /**
+   * Alias for stop
+   */
+  cancel ( ) { this.stop(); }
+
   applyTargetUpdate (current) {
     if (typeof this._target === 'function') {
       this._target(current);