use env variable for host in swagger_inject

[mlankenau]

I really enjoy using maru in combination with maru_swagger. Great library!!
But I have a minor issue: I would like to use an environment var for the host (that is put into the swagger json). doing something like
host: System.get_env("HOST") || "localhost"
does not work. I guess the get_env will be called at compile time, since it is a macro.
Is there a known workaround?

Cheers
Marcus