feature: items和value支持自动watch

src/core/4.widget.js

@@ -241,7 +241,9 @@
                 }, (handler && function (v) {
                     handler.call(self, self, v);
                 }) || BI.emptyFn, BI.extend({deep: true}, options));
-                this._watchers.push(watcher);
+                this._watchers.push(function unwatchFn() {
+                    watcher.teardown();
+                });
                 return watcher.value;
             } else {
                 return getter();