Support a way to trigger MVVM command silently

Sometimes we want a Form binding to save the data before or after some commands and these commands might not appear in a ViewModel object, so if there is a way to ignore the binder implementation throwing an exception.

For example,

// zul
<grid form="@id('fx') @load(vm.currentBook) @save(vm.currentBook, before='save')"/>

// java
// @Command("save")
// public void doSave() {}