Update to es6 modules and more modern js syntax

Fixes #104

Author: jasongrout

{{cookiecutter.github_project_name}}/js/amd-public-path.js

@@ -1,8 +1,8 @@
 // In an AMD module, we set the public path using the magic requirejs 'module' dependency
 // See https://github.com/requirejs/requirejs/wiki/Differences-between-the-simplified-CommonJS-wrapper-and-standard-AMD-define#module
 // Since 'module' is a requirejs magic module, we must include 'module' in the webpack externals configuration.
-var module = require('module');
-var url = new URL(module.uri, document.location)
+import * as module from 'module';
+const url = new URL(module.uri, document.location)
 // Using lastIndexOf('/')+1 gives us the empty string if there is no '/', so pathname becomes '/'
 url.pathname = url.pathname.slice(0,url.pathname.lastIndexOf('/')+1);
 __webpack_public_path__ = `${url.origin}${url.pathname}`;

{{cookiecutter.github_project_name}}/js/lib/example.js

@@ -1,9 +1,7 @@
-var widgets = require('@jupyter-widgets/base');
-var _ = require('lodash');
+import { DOMWidgetModel, DOMWidgetView } from '@jupyter-widgets/base';
 
 // See example.py for the kernel counterpart to this file.
 
-
 // Custom Model. Custom widgets models must at least provide default values
 // for model attributes, including
 //
@@ -18,38 +16,32 @@ var _ = require('lodash');
 //  when different from the base class.
 
 // When serialiazing the entire widget state for embedding, only values that
-// differ from the defaults will be specified.
-var HelloModel = widgets.DOMWidgetModel.extend({
-    defaults: _.extend(widgets.DOMWidgetModel.prototype.defaults(), {
+// differ from the defaults will be serialized.
+
+export class HelloModel extends DOMWidgetModel {
+    defaults() {
+      return {
+        ...super.defaults(),
         _model_name : 'HelloModel',
         _view_name : 'HelloView',
         _model_module : '{{ cookiecutter.npm_package_name }}',
         _view_module : '{{ cookiecutter.npm_package_name }}',
         _model_module_version : '{{ cookiecutter.npm_package_version }}',
         _view_module_version : '{{ cookiecutter.npm_package_version }}',
         value : 'Hello World!'
-    })
-});
-
+      };
+    }
+  }
 
-// Custom View. Renders the widget model.
-var HelloView = widgets.DOMWidgetView.extend({
-    // Defines how the widget gets rendered into the DOM
-    render: function() {
+export class HelloView extends DOMWidgetView {
+    render() {
         this.value_changed();
 
-        // Observe changes in the value traitlet in Python, and define
-        // a custom callback.
+        // Observe and act on future changes to the value attribute
         this.model.on('change:value', this.value_changed, this);
-    },
+    }
 
-    value_changed: function() {
+    value_changed() {
         this.el.textContent = this.model.get('value');
     }
-});
-
-
-module.exports = {
-    HelloModel: HelloModel,
-    HelloView: HelloView
-};
+}

{{cookiecutter.github_project_name}}/js/lib/extension.js

@@ -13,7 +13,4 @@ if (window.require) {
     });
 }
 
-// Export the required load_ipython_extension
-module.exports = {
-    load_ipython_extension: function() {}
-};
+export function load_ipython_extension() { };

{{cookiecutter.github_project_name}}/js/lib/index.js

@@ -1,3 +1,4 @@
 // Export widget models and views, and the npm package version number.
-module.exports = require('./example.js');
-module.exports['version'] = require('../package.json').version;
+
+export {HelloModel, HelloView} from './example';
+export {version} from '../package.json';

{{cookiecutter.github_project_name}}/js/lib/labplugin.js

@@ -1,16 +1,17 @@
-var plugin = require('./index');
-var base = require('@jupyter-widgets/base');
+import {HelloModel, HelloView, version} from './index';
+import {IJupyterWidgetRegistry} from '@jupyter-widgets/base';
 
-module.exports = {
+export const helloWidgetPlugin = {
   id: '{{ cookiecutter.npm_package_name }}:plugin',
-  requires: [base.IJupyterWidgetRegistry],
+  requires: [IJupyterWidgetRegistry],
   activate: function(app, widgets) {
       widgets.registerWidget({
           name: '{{ cookiecutter.npm_package_name }}',
-          version: plugin.version,
-          exports: plugin
+          version: version,
+          exports: { HelloModel, HelloView }
       });
   },
   autoStart: true
 };
 
+export default helloWidgetPlugin;

{{cookiecutter.github_project_name}}/js/webpack.config.js

@@ -1,15 +1,15 @@
-var path = require('path');
-var version = require('./package.json').version;
+const path = require('path');
+const version = require('./package.json').version;
 
 // Custom webpack rules are generally the same for all webpack bundles, hence
 // stored in a separate local variable.
-var rules = [
+const rules = [
     { test: /\.css$/, use: ['style-loader', 'css-loader']}
 ]
 
 
 module.exports = (env, argv) => {
-    var devtool = argv.mode === 'development' ? 'source-map' : false;
+    const devtool = argv.mode === 'development' ? 'source-map' : false;
     return [
         {// Notebook extension
         //