Include sourceMapContents by default if sourceMaps are wanted

This way the developer does not have to map the local path inside Chrome, because the contents are delivered directly with the sourceMap.

README.md

@@ -131,7 +131,7 @@ module.exports = {
 };
 ```
 
-If you want to view the original Sass files inside Chrome and even edit it,  [there's a good blog post](https://medium.com/@toolmantim/getting-started-with-css-sourcemaps-and-in-browser-sass-editing-b4daab987fb0). Checkout [test/sourceMap](https://github.com/jtangelder/sass-loader/tree/master/test) for a running example. Make sure to serve the content with an HTTP server.
+If you want to edit the original Sass files inside Chrome, [there's a good blog post](https://medium.com/@toolmantim/getting-started-with-css-sourcemaps-and-in-browser-sass-editing-b4daab987fb0). Checkout [test/sourceMap](https://github.com/jtangelder/sass-loader/tree/master/test) for a running example. Make sure to serve the content with an HTTP server.
 
 ## License
 

index.js

@@ -135,6 +135,12 @@ module.exports = function (content) {
         // however, it is still necessary for correct relative paths in result.map.sources
         opt.sourceMap = this.options.output.path + '/sass.map';
         opt.omitSourceMapUrl = true;
+
+        // If sourceMapContents option is not set, set it to true otherwise maps will be empty/null
+        // when exported by webpack-extract-text-plugin.
+        if ('sourceMapContents' in opt === false) {
+            opt.sourceMapContents = true;
+        }
     }
 
     // indentedSyntax is a boolean flag