vue 三级导航菜单

<el-submenu index='1-4'>

<tmplate slot='title'>选项4</tmplate>

<el-submenu index='1-4-1'>

<tmplate slot='title'>选项4-1</tmplate>

<el-submenu index='1-4-1-1'>

<tmplate slot='title'>选项4-1-1</tmplate>

<el-menu-item index='1-4-1'>选项1</el-menu-item>

</el-submenu>

</el-submenu>

</el-submenu>