Update webpack config for use with js demo

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

@@ -7,35 +7,51 @@ module.exports = (env, argv) => {
 
     let mode;
 
+    const overrides = module.exports || {};
 
     // if user specified mode flag take that value
     if (argv && argv.mode) {
         mode = argv.mode;
     }
 
     // else if configuration object is already set (module.exports) use that value
-    else if (module.exports && module.exports.mode) {
-        mode = module.exports = mode;
+    else if (overrides.mode) {
+        mode = overrides.mode;
     }
 
-    // else take webpack default
+    // else take webpack default (production)
     else {
-        mode = 'production'; // webpack default
+        mode = 'production';
     }
 
+    let filename = (overrides.output || {}).filename;
+    if(!filename) {
+        const modeSuffix = mode === 'development' ? 'dev' : 'min';
+        filename = `${dashLibraryName}.${modeSuffix}.js`;
+    }
+
+    const entry = overrides.entry || {main: './src/lib/index.js'};
+
+    const devtool = overrides.devtool || (
+        mode === 'development' ? "eval-source-map" : 'none'
+    );
+
+    const externals = ('externals' in overrides) ? overrides.externals : ({
+        react: 'React',
+        'react-dom': 'ReactDOM',
+        'plotly.js': 'Plotly',
+    });
+
     return {
-        entry: {main: './src/lib/index.js'},
+        mode,
+        entry,
         output: {
             path: path.resolve(__dirname, dashLibraryName),
-            filename: `${dashLibraryName}.${mode === 'development' ? 'dev' : 'min'}.js`,
+            filename,
             library: dashLibraryName,
             libraryTarget: 'window',
         },
-        externals: {
-            react: 'React',
-            'react-dom': 'ReactDOM',
-            'plotly.js': 'Plotly',
-        },
+        externals,
         module: {
             rules: [
                 {
@@ -58,6 +74,6 @@ module.exports = (env, argv) => {
                 },
             ],
         },
-        devtool: mode === 'development' ? "eval-source-map" : 'none'
+        devtool
     }
 };