瀑布流加载功能实现 && 文档补全

docs/examples/waterfall.md

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+<script>
+export default {
+  data() {
+    return {
+      list: [1, 2, 3, 4, 5],
+      loading: false,
+      finished: false
+    };
+  },
+  methods: {
+    loadMore() {
+      if (this.list.length >= 200) {
+        this.finished = true;
+        return;
+      }
+
+      this.loading = true;
+      setTimeout(() => {
+        let lastNumber = this.list[this.list.length - 1];
+        for (let i = 0; i < 5; i ++) {
+          lastNumber += 1;
+          this.list.push(lastNumber);
+        }
+        this.loading = false;
+      }, 2500);
+    },
+    loadMoreUpper() {
+      if (this.list[0] < 0) return;
+      this.list.unshift(-1);
+    }
+  },
+  computed: {
+    isWaterfallDisabled: function() {
+      return this.loading || this.finished;
+    }
+  }
+};
+</script>
+<style lang="css">
+  .waterfall {
+    height: 300px;
+    overflow: scroll;
+  }
+  .waterfall-item {
+    line-height: 20px;
+    padding: 5px 0;
+  }
+</style>
+
+## Waterfall组件
+
+### 基础用法
+
+:::demo 样例代码
+```html
+<div class="waterfall">
+  <div
+    v-waterfall-lower="loadMore"
+    v-waterfall-upper="loadMoreUpper"
+    waterfall-disabled="isWaterfallDisabled"
+    waterfall-offset="400"
+  >
+    <div
+      class="waterfall-item"
+      v-for="item in list"
+      style="text-align: center;"
+    >
+      {{ item }}
+    </div>
+    <div v-if="loading" style="text-align: center;">
+      loading
+    </div>
+  </div>
+</div>
+```
+:::
+
+### API
+
+| 参数       | 说明      | 类型       | 默认值       | 可选值       |
+|-----------|-----------|-----------|-------------|-------------|
+| waterfall-disabled | 是否禁止瀑布流触发 | Boolean  | false |  |
+| waterfall-offset | 触发瀑布流加载的阈值 | Number  | 300 |   |
+