Feature request: add `postprocessor` to `RunOptions`

[kunaakos]

Using a postprocessor function is documented, but AFAICT it's not possible to use this feature by calling the run function exported by the module.

Adding this as an optional parameter to RunOptions should be easy and very useful.

I'd be happy to have a go at this, in case I'm not missing something and it's possible to do this already.