Built-in reverse proxy now supports rewriting headers

This is necessary for using Keter as a front-end to other web
services that may require hacks like Jenkins does, where the
"Location" header needs to be rewritten to use https on every
response.

Example:

```
root: ..
port: 80
ssl:
  # host:
  port:
  key:
  certificate:
reverse-proxy:
  - reversed-host: jenkins-internal.corp.example.com
    reversed-port: 8080
    reversing-host: jenkins.example.com
    ssl: False
    rewrite-response:
      - header: Location
        from: ^http://jenkins.example.com
        to: https://jenkins.example.com
```