feat: add umi/prompt (umijs#1267)

e.g.

```js
import Prompt from 'umi/prompt';

export default () => {
  return (
    <>
      <h1>Prompt</h1>
      <Prompt
        when={true}
        message={(location) => {
          return window.confirm(`confirm to leave to ${location.pathname}?`);
        }}
      />
    </>
  );
}

```

docs/api/README.md

@@ -102,7 +102,31 @@ import Redirect from 'umi/redirect';
 <Redirect to="/login" />
 ```
 
-Detail: [https://reacttraining.com/react-router/web/api/Redirect](https://reacttraining.com/react-router/web/api/Redirect)
+See: [https://reacttraining.com/react-router/web/api/Redirect](https://reacttraining.com/react-router/web/api/Redirect)
+
+### umi/prompt
+
+Example.
+
+```js
+import Prompt from 'umi/prompt';
+
+export default () => {
+  return (
+    <>
+      <h1>Prompt</h1>
+      <Prompt
+        when={true}
+        message={(location) => {
+          return window.confirm(`confirm to leave to ${location.pathname}?`);
+        }}
+      />
+    </>
+  );
+}
+```
+
+See：[https://reacttraining.com/react-router/web/api/Prompt](https://reacttraining.com/react-router/web/api/Prompt)
 
 ### umi/withRouter
 

docs/zh/api/README.md

@@ -104,6 +104,30 @@ import Redirect from 'umi/redirect';
 
 详见：[https://reacttraining.com/react-router/web/api/Redirect](https://reacttraining.com/react-router/web/api/Redirect)
 
+### umi/prompt
+
+例子：
+
+```js
+import Prompt from 'umi/prompt';
+
+export default () => {
+  return (
+    <>
+      <h1>Prompt</h1>
+      <Prompt
+        when={true}
+        message={(location) => {
+          return window.confirm(`confirm to leave to ${location.pathname}?`);
+        }}
+      />
+    </>
+  );
+}
+```
+
+详见：[https://reacttraining.com/react-router/web/api/Prompt](https://reacttraining.com/react-router/web/api/Prompt)
+
 ### umi/withRouter
 
 详见：[https://reacttraining.com/react-router/web/api/withRouter](https://reacttraining.com/react-router/web/api/withRouter)

packages/umi-build-dev/src/plugins/afwebpack-config.js

@@ -78,6 +78,7 @@ export default function(api) {
       .set('umi/dynamic', join(process.env.UMI_DIR, 'lib/dynamic.js'))
       .set('umi/navlink', join(process.env.UMI_DIR, 'lib/navlink.js'))
       .set('umi/redirect', join(process.env.UMI_DIR, 'lib/redirect.js'))
+      .set('umi/prompt', join(process.env.UMI_DIR, 'lib/prompt.js'))
       .set('umi/router', join(process.env.UMI_DIR, 'lib/router.js'))
       .set('umi/withRouter', join(process.env.UMI_DIR, 'lib/withRouter.js'))
       .set(

packages/umi/src/prompt.js

@@ -0,0 +1,2 @@
+import { Prompt } from 'react-router-dom';
+export default Prompt;