Make sourceURL unique (#44)

The [sourceMap 3 sepcification](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit) mentions
sourceURL is described [here](http://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/)
, which mentions that
> The name has to uniquely identify the buffer

Where `map.file` is currently not, so for example if I have multiple `style.scss` files in my project,
clicking on chrome's inspector's diplayed filename will always take me to some specific one and not the correct one.
( mabye the first one, not sure. )

src/insertCss.js

@@ -82,7 +82,7 @@ function insertCss(styles, options) {
     if (sourceMap) {
       cssText += `\n/*# sourceMappingURL=data:application/json;base64,${
         b64EncodeUnicode(JSON.stringify(sourceMap))}*/`;
-      cssText += `\n/*# sourceURL=${sourceMap.file}*/`;
+      cssText += `\n/*# sourceURL=${id}*/`;
     }
 
     if ('textContent' in elem) {