Modify strm / set level per file?

[nigoroll]

After having let it sit in a toy example state for a year, I do now intend to turn the varnish-cache zipflow module into something useful. In this context, it would be helpful if the level could be adjusted per file, and I would like to ask for your advice on how this could be achieved. Would it make sense to expose the z_stream via a function such that deflate* (deflateParams in particular) could be used or would a different approach be more sensible?

Based on your feedback, I would volunteer to propose a PR if that was considered helpful.