[new feature] add Collapse component (#674)

docs/markdown/en-US/collapse.md

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+## Collapse
+
+### Install
+``` javascript
+import { Collapse } from 'vant';
+
+Vue.use(Collapse);
+```
+
+### Usage
+
+#### Basic Usage
+Use `v-model` to control the name of active panels
+
+```html
+<van-collapse v-model="activeNames">
+  <van-collapse-item title="Title1" name="1">Content</van-collapse-item>
+  <van-collapse-item title="Title2" name="2">Content</van-collapse-item>
+  <van-collapse-item title="Title3" name="3">Content</van-collapse-item>
+</van-collapse>
+```
+
+``` javascript
+export default {
+  data() {
+    return {
+      activeNames: ['1']
+    };
+  }
+};
+```
+
+#### Accordion
+In accordion mode, only one panel can be expanded at the same time.
+
+```html
+<van-collapse v-model="activeName" accordion>
+  <van-collapse-item title="Title1" name="1">Content</van-collapse-item>
+  <van-collapse-item title="Title2" name="2">Content</van-collapse-item>
+  <van-collapse-item title="Title3" name="3">Content</van-collapse-item>
+</van-collapse>
+```
+
+``` javascript
+export default {
+  data() {
+    return {
+      activeName: '1'
+    };
+  }
+};
+```
+
+#### Custom title
+
+```html
+<van-collapse v-model="activeNames">
+  <van-collapse-item name="1">
+    <div slot="title">Title1<van-icon name="question" /></div>
+    Content
+  </van-collapse-item>
+  <van-collapse-item title="Title2" name="2">
+    Content
+  </van-collapse-item>
+</van-collapse>
+```
+
+
+
+### Collapse API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| v-model | names of current active panels | `Array | String | Number` | - | - |
+| accordion | Whether to be accordion mode | `Boolean` | `false` | - |
+
+### Collapse Event
+
+| Event | Description | Arguments |
+|-----------|-----------|-----------|
+| change | Triggered when switch panel | activeNames: `string | array` |
+
+### CollapseItem API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| name | Name | `String | Number` | `index` | - |
+| title | Title | `String` | - | - |
+
+### CollapseItem Slot
+
+| name | Description |
+|-----------|-----------|
+| default | Content |
+| title | Custom title |