[binder] add extension that binds to a specific emitter

src/utils/binder_for.js

@@ -0,0 +1,12 @@
+import { Binder } from './';
+
+export default class BinderFor extends Binder {
+  constructor(emitter) {
+    super();
+    this.emitter = emitter;
+  }
+
+  on(...args) {
+    super.on(this.emitter, ...args);
+  }
+}

src/utils/index.js

@@ -0,0 +1,3 @@
+export Binder from './binder';
+export BinderFor from './binder_for';
+export fromRoot from './fromRoot';