feat: add demo with dependencies (#27)

+export default {
+  plugins: [
+    ['umi-plugin-block-dev', {
+      layout: 'ant-design-pro',
+    }],
+    ['umi-plugin-react', {
+      antd: true,
+    }],
+  ],
+}

+{
+  "name": "@umi-blocks/demo-with-dependencies",
+  "description": "a demo block with dependencies",
+  "scripts": {
+    "dev": "umi dev"
+  },
+  "dependencies": {
+    "antd": "^3.10.9"
+  },
+  "devDependencies": {
+    "umi": "^2.3.0",
+    "umi-plugin-react": "^1.3.0",
+    "umi-plugin-block-dev": "^1.0.0"
+  },
+  "blockConfig": {
+    "dependencies": ["demo"]
+  }
+}

+import React from 'react';
+import { Row, Col } from 'antd';
+import Demo from './Demo';
+import styles from './index.less';
+
+export default () => {
+  return (
+    <div className={styles.container}>
+      <Row><Col span={24}>I am a block with dependencies.</Col></Row>
+      <Row>
+        <Col span={12}><Demo /></Col>
+        <Col span={12}><Demo /></Col>
+      </Row>
+    </div>
+  )
+}

+.container {
+  color: red;
+}
\ No newline at end of file

-import styles from './index.less';
 import React from 'react';
 import { Button } from 'antd';
+import styles from './index.less';
 
 export default () => {
   return <Button className={styles.container}>Hello UmiJS!</Button>