reverse proxy: support URL rewriting

[kevinbader]

Right now, {id} can be used as a placeholder for variable parts of an URL. There are issues with the current approach:

• The name inside the curly braces does not matter - it can't be referred to in the target URL.
• The format is non-standard and quite inflexible.

I think that if we want to support URL rewriting we should adopt industry standards, which basically means using regular expressions. Using regular expressions is admittedly a bit more complicated than the current approach might be, but it is very efficient (expressions can be pre-compiled) and flexible.

The implementation should enable the following scenario:

Given the request URL `/books/123/chapters/456`,
and the rule `/books/([^/]+)/chapters(?<chapter_id>/?|/.*)` => `http://chapter-service:8080/v1/chapters/chapter_id`,
then the request is forwarded to `http://chapter-service:8080/v1/chapters/456`.

Proposed :re options: :no_auto_capture, :dupnames, :unicode, :anchored, :caseless.