Vue.directive('xxx-xxx', function(el, binding){
       el.style.backgroundColor = binding.value ;
    })
    //v-bind及v-on的简写
    <!--完整语法-->
    <a v-bind:href="url">bind</a>
    <!--缩写-->
    <a :href="url">bind</a>
    <!--完整语法-->
    <a v-on:click="doSomething">on</a>
    <!--缩写-->
    <a @click="doSomething">on</a>