Ability to use variables in replacement strings for rewrite middleware.

I was actually thinking about rolling back all these changes I made to `vulcan` library altogether, for now, cause they turned out to be quite pointless.

It makes sense to move this "templating" thing into a separate package under `vulcan` library, make it more generic, rather than just have it inside rewrite middleware.

I think I will finish with rewrite first (e.g. response rewriting) cause it kinda blocks me and then will look into it.