changed build config to 'source-map' devtool

Author: liabru

webpack.demo.config.js

@@ -29,6 +29,7 @@ License ${pkg.license}`;
     return {
         entry: { [name]: './demo/src/index.js' },
         node: false,
+        devtool: 'source-map',
         output: {
             library: 'MatterDemo',
             libraryTarget: 'umd',