Support a suffix for compressed files

In our use case the server would want to know if a file is compressed or not so that it can serve the file in a way that the browser understands. One way of accomplishing this would be to add a `outputSuffix` parameter that, when set, would write the compressed version as `filename+outputSuffix` instead of overwriting the original file.

If this makes sense, I could create a PR once #11 is resolved somehow