Issue title: Support [path] in the output path.
 Issue body: That's it, I just think this plugin should support `[path]` in the output path in case you want to keep your assets where they came from.
 Comments: 
 Comment 0: I just come into the same situation that I would like the css file being generated in another folder

sorry for my poor English

 Comment 1: ETWP entered maintenance mode and won't receive any feature updates, please open an issue in [`mini-css-extract-plugin`](https://github.com/webpack-contrib/mini-css-extract-plugin) instead

It's not possible to support anyways :)
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