Include variables in url_prefix

[fchambo]

It would be great if we could make url_prefix a bit more dynamic.

I will try to explain my scenario, please ask for clarifications if necessary:

Sinopia server: available from http://mycompany.com/sinopia and http://10.1.1.1/sinopia via nginx.

Client A: it is inside Sinopia server's network (ip: 10.1.1.2) but it has no access to http://mycompany.com/sinopia due to firewall restrictions.

Client B: it is outside Sinopia server's network, so it has no access to 10.1.1.1, but it has access through http://mycompany.com/sinopia.

So, if I configure url_prefix: http://mycompany.com/sinopia, then Client A throws timeout because sinopia change links with mycompany.com host. And if I configure url_prefix: http://10.1.1.1/sinopia, then Client B throws timeout because sinopia change links with 10.1.1.1 host.

What I propose is to add the feature to add some simple variables to url_prefix so it would be possible to configure something like url_prefix: $scheme://$host:$port/sinopia

I think that with those 3 simple variables we open a big range of new configuration possibilities. I will be working on a pull request within this days, but I would also like to know your opinions on this new feature.