[new feature] add Collapse component (#674)

packages/collapse/index.vue

@@ -0,0 +1,41 @@
+<template>
+  <div class="van-collapse van-hairline--top-bottom">
+    <slot />
+  </div>
+</template>
+
+<script>
+import { create } from '../utils';
+
+export default create({
+  name: 'van-collapse',
+
+  model: {
+    prop: 'activeNames'
+  },
+
+  props: {
+    accordion: Boolean,
+    activeNames: [String, Number, Array]
+  },
+
+  data() {
+    return {
+      items: []
+    };
+  },
+
+  methods: {
+    switch(name, expanded) {
+      const { activeNames } = this;
+      if (!this.accordion) {
+        name = expanded
+          ? activeNames.concat(name)
+          : activeNames.filter(activeName => activeName !== name);
+      }
+      this.$emit('change', name);
+      this.$emit('input', name);
+    }
+  }
+});
+</script>