add Dropdown component

add Dropdown component

test/routers/dropdown.vue

@@ -0,0 +1,33 @@
+<style>
+    body{
+        padding: 50px;
+    }
+</style>
+<template>
+    <Dropdown trigger="click" align="right" @on-click="click">
+        <i-button type="primary">
+            下拉菜单
+            <Icon type="arrow-down-b"></Icon>
+        </i-button>
+        <Dropdown-menu slot="list">
+            <Dropdown-item>张三</Dropdown-item>
+            <Dropdown-item disabled>李四</Dropdown-item>
+            <Dropdown-item>王五</Dropdown-item>
+            <Dropdown-item divided>周六</Dropdown-item>
+        </Dropdown-menu>
+    </Dropdown>
+</template>
+<script>
+    export default {
+        props: {},
+        data () {
+            return {}
+        },
+        computed: {},
+        methods: {
+            click (key) {
+                console.log(key)
+            }
+        }
+    }
+</script>