Allow before/after streams for transforming the request

[jcrugzz]

Based around the concept I outlined here. This will allow arbitrary transformations of the request being proxied as well as the response we get back.

[smazurov]

👍

[philippotto]

I wrote a very simple middleware which can be plugged into http-proxy. It allows modifying the proxied data by providing a simple function. It is called transformer-proxy. Maybe someone finds it useful.

The modifyResponse-middleware.js-example didn't work for me, because the data is passed in chunks and not as a whole and the content-length becomes wrong when the data length is changed.