allow chaning service endpoint inside the middleware

[mikz]

• added request.endpoint which carries the service endpoint
• brainslug middleware uses request.endpoint to make the final request

I'm not very happy with the request.endpoint. I would prefer some
global object, a service for example. So it would be service.endpoint.

But it is much harder to implement. Will give it some time to rest and
see.

allows to change endpoint on the fly in the middleware like:

return function(request, next_middleware)
  if request.path == '/some/path' then
    request.endpoint = 'https://example.com'
  end
  return next_middleware()
end

fixes #18

[galenzhao]

very useful
please merge to master

[mikz]

@galenzhao have you tried it?

Even if I merge it to master, I'm not sure when I'll be able to do a release.

The release happens from on-premise branch anyway.