Export a combohandler wrapper function to (re-)enable `app.use()` pattern.

The goal is to fix #23, which it turns out is greatly aided by unit tests that actually test the pattern we claimed to support. :)

Tasks still remaining:
- [ ] Tests for CSS `url()` rewriting in `app.use()` cases.
- [ ] Refinement of `README` docs for this pattern.
- [ ] Noodling about "enable this on all routes" recommendation.
- [ ] Update `HISTORY.md`