The way I like to do it is to pass a module on the command line
that contains the custom config. So in the app:
---
import myapp.config;
// use the variables defined in there like normal
---
Now, to define a config file, you do something like:
myconfiguration.d # note that the file name can be anything!
---
module myapp.config; // but each must use this same module config
enum some_key = "some_value";
// and so on
---
Now, when you compile, you build it with a particular config by
passing one of those files to the compile:
dmd myapp.d myconfiguration.d # or whatever single config you want
Then you can define as much as you want in the config module, and
have as many of them as you want too.
