Add possibility to extract file outside of directory with bundle

Hey 👋

Current css extraction can't extract css file outside of directory with created bundle.

If you are lucky to use `output.file` with path `js/app.js`, it's impossible to move css file outside of `js` folder. Workarounds from https://github.com/egoist/rollup-plugin-postcss/issues/250 and https://github.com/egoist/rollup-plugin-postcss/issues/265 where you have to use copy plugin and rimraf are plain horrible.

There has to be a normal way of doing such a simple thing, be it emitFile removal, addition of option or api to opt out emitFile or something else.

Don't close this one until it's fixed please, let's gather ideas and find a solution.

rollup: 2.15.0
rollup-plugin-postcss: 3.1.2