feat: add column desc api and example (#2)

Co-authored-by: GU Yiling <justice360@gmail.com>

one/docs/demo/table/desc.vue

@@ -0,0 +1,93 @@
+<template>
+<article>
+  <section>
+    <veui-table
+      :ui="ui"
+      :data="data"
+      key-field="id"
+    >
+      <veui-table-column
+        field="id"
+        title="ID"
+      />
+      <veui-table-column
+        field="name"
+        title="Name"
+        :desc="nameDesc"
+      />
+      <veui-table-column
+        field="bid"
+        title="Bid"
+        width="160"
+        align="right"
+      >
+        <template slot-scope="{ bid }">
+          {{ bid | currency }}
+        </template>
+        <template #desc="{ close }">
+          <p>This is a description for bid.</p>
+          <veui-button @click="close">close</veui-button>
+        </template>
+      </veui-table-column>
+    </veui-table>
+  </section>
+</article>
+</template>
+
+<script>
+import { Table, Column, Button } from 'veui'
+
+export default {
+  components: {
+    'veui-table': Table,
+    'veui-table-column': Column,
+    'veui-button': Button
+  },
+  filters: {
+    currency (value) {
+      return '¥' + value.toFixed(2)
+    }
+  },
+  data () {
+    return {
+      data: [
+        {
+          id: '31541',
+          name: 'Steve Rogers',
+          bid: 1024,
+          updateDate: '20131117'
+        },
+        {
+          id: '31552',
+          name: 'Thor Odinson',
+          bid: 598,
+          updateDate: '20140214'
+        },
+        {
+          id: '31563',
+          name: 'Tony Stark',
+          bid: 820,
+          updateDate: '20170610'
+        },
+        {
+          id: '31574',
+          name: 'Stephen Strange',
+          bid: 736,
+          updateDate: '20180109'
+        }
+      ],
+      nameDesc: 'This is a description for name.'
+    }
+  }
+}
+</script>
+
+<style lang="less" scoped docs>
+section {
+  margin-bottom: 20px;
+}
+
+h4 {
+  margin-top: 0;
+}
+</style>