Add option to make dest path relative to source path

[nirazul]

Thanks for creating this plugin!

I've fiddled around with it a bit. Unfortunately, there's no option to set the output path to something relative to the input stylesheet. Technically, this should be possible by reading css.source.input.file.

Even better would be adding a new file to the stream to allow plugins that come after postcss (for example in a gulp file) to simultaneously treat both the main file and the extracted file.

[Nitive]

Do you want creating a PR for this?

[nirazul]

I can do that, at least the relative path thing. When it comes to streams, I'm less sure 😁