使用element-ui组件后原@keyup.enter事件无效：
<el-input type="text" v-model="item"
placeholder="what needs to be done?"
@keyup.enter='addtodo'></el-input>
应加上.native覆盖原有封装的@keyup事件：
<el-input type="text" v-model="item"
placeholder="what needs to be done?"
@keyup.native.13='addtodo'></el-input>
13是回车键 其他键也可以用数字代替
