css sourceMappingURL is broken

I'm trying to use this plugin with CSS source maps, but I can't get it to work in any fashion because this plugin doesn't understand the double extension. With this config:

``` js
tree = assetRev(tree, {
extensions: ['js', 'css', 'png', 'jpg', 'gif', 'map'],
exclude: ['fonts/169929'],
replaceExtensions: ['html', 'js', 'css'],
});
```

I get this css file:

``` css
/*# sourceMappingURL=main-aafff1edf19e012de9347224d8cd4856.css-e997028ad01268b4cda33c1c4ce89b36.map */
```

but the source-map is renamed to `main.css-e997028ad01268b4cda33c1c4ce89b36.map`

Other combinations have similar results - even ignoring `css.map` doesn't help.