feat: Add preRewrite and routes options.

[ShogunPanda]

This PR adds two new options:

1. routes, which is used to explicitly provide the list of routes we want to manage via http-proxy. Defaults to / and /*, which is the current behavior.
2. preRewrite, which is a function which can be provided to manipulate the request URL before rewriting and serving it via reply.from.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm