feat(async/withScriptjs): provide HOC for loading googleMapURL with scriptjs

* `withScriptjs` requires a prop: `googleMapURL`
* `withScriptjs` requires a prop: `loadingElement`

BREAKING CHANGE: Apply `withScriptjs` HOC before `withGoogleMaps` HOC.

Before:

```js
<ScriptjsLoader
  hostname={"maps.googleapis.com"}
  pathname={"/maps/api/js"}
  query={{v: `3.${ AsyncGettingStarted.version }`, libraries: "geometry,drawing,places"}}
  loadingElement={
    <div {...this.props} style={{ height: "100%" }}>
      <FaSpinner />
    </div>
  }
  containerElement={
    <div {...this.props} style={{ height: "100%" }} />
  }
  googleMapElement={
    <GoogleMap
      ref={googleMap => {
        googleMap && console.log(`Zoom: ${ googleMap.getZoom() }`);
      }}
      defaultZoom={3}
      defaultCenter={{lat: -25.363882, lng: 131.044922}}
      onClick={::this.handleMapClick}
    >
      <Marker
        {...this.state.marker}
        onRightclick={this.handleMarkerRightclick}
      />
    </GoogleMap>
  }
/>
```

After:

```js
// Wrap all `react-google-maps` components with `withGoogleMap` HOC
// then wraps it into `withScriptjs` HOC
// It loads Google Maps JavaScript API v3 for you asynchronously.
// Name the component AsyncGettingStartedExampleGoogleMap
const AsyncGettingStartedExampleGoogleMap = withScriptjs(
  withGoogleMap(
    props => (
    <GoogleMap
      ref={props.onMapLoad}
      defaultZoom={3}
      defaultCenter={{ lat: -25.363882, lng: 131.044922 }}
      onClick={props.onMapClick}
    >
      {props.markers.map(marker => (
        <Marker
          {...marker}
          onRightClick={() => props.onMarkerRightClick(marker)}
        />
      ))}
    </GoogleMap>
  )
);
// Then, render it:
render(
  <GettingStartedGoogleMap
    googleMapURL="https://maps.googleapis.com/maps/api/js?v=3.exp"
    loadingElement={
      <div style={{ height: `100%` }}>
        <FaSpinner
          style={{
            display: `block`,
            width: `80px`,
            height: `80px`,
            margin: `150px auto`,
            animation: `fa-spin 2s infinite linear`,
          }}
        />
      </div>
    }
    containerElement={
      <div style={{ height: `100%` }} />
    }
    mapElement={
      <div style={{ height: `100%` }} />
    }
    onMapLoad={_.noop}
    onMapClick={_.noop}
    markers={markers}
    onMarkerRightClick={_.noop}
  />,
  document.getElementById('root')
);
```

Author: tomchentw

src/lib/async/ScriptjsGoogleMap.js

@@ -0,0 +1,98 @@
+import _ from "lodash";
+
+import invariant from "invariant";
+
+import canUseDOM from "can-use-dom";
+
+import getDisplayName from "react-display-name";
+
+import {
+  default as React,
+  PropTypes,
+  Component,
+} from "react";
+
+const LOAIDNG_STATE_NONE = `NONE`;
+const LOAIDNG_STATE_BEGIN = `BEGIN`;
+const LOAIDNG_STATE_LOADED = `LOADED`;
+
+export default function withScriptjs(WrappedComponent) {
+  return class Container extends Component {
+    static displayName = `withScriptjs(${getDisplayName(WrappedComponent)})`;
+
+    static propTypes = {
+      loadingElement: PropTypes.node.isRequired,
+      googleMapURL: PropTypes.string.isRequired,
+    };
+
+    state = {
+      loadingState: LOAIDNG_STATE_NONE,
+    };
+
+    isUnmounted = false;
+
+    handleLoaded = this.handleLoaded.bind(this);
+
+    handleLoaded() {
+      if (this.isUnmounted) {
+        return;
+      }
+      this.setState({
+        loadingState: LOAIDNG_STATE_LOADED,
+      });
+    }
+
+    componentWillMount() {
+      const {
+        loadingElement,
+        googleMapURL,
+      } = this.props;
+      invariant(!!loadingElement && !!googleMapURL,
+`Required props loadingElement or googleMapURL is missing. You need to provide both of them.`
+      );
+    }
+
+    componentDidMount() {
+      const {
+        loadingState,
+      } = this.state;
+      if (loadingState !== LOAIDNG_STATE_NONE || !canUseDOM) {
+        return;
+      }
+      this.setState({
+        loadingState: LOAIDNG_STATE_BEGIN,
+      });
+      // Don't load scriptjs as dependency since we want this module be used on server side.
+      // eslint-disable-next-line global-require
+      const scriptjs = require(`scriptjs`);
+      const {
+        googleMapURL,
+      } = this.props;
+      scriptjs(googleMapURL, this.handleLoaded);
+    }
+
+    componentWillUnmount() {
+      this.isUnmounted = true;
+    }
+
+    render() {
+      const {
+        loadingElement,
+        googleMapURL, // eslint-disable-line no-unused-vars
+        ...restProps,
+      } = this.props;
+
+      const {
+        loadingState,
+      } = this.state;
+
+      if (loadingState === LOAIDNG_STATE_LOADED) {
+        return (
+          <WrappedComponent {...restProps} />
+        );
+      } else {
+        return loadingElement;
+      }
+    }
+  };
+}