Use Configy to read configuration, let env override config

[Geod24]

Normally, the environment variables always take preference on the configuration.
Hence, changing the priority here makes sense. Additionally, Dub already vendors
Configy, which we can just use instead of manual parsing.

[s-ludwig]

Is the @Name something provided by Configy? Or more importantly, does it expect the camelCase names in the JSON file, or the @Name ones? It's not really visible from the source code now, so at least a comment, or better yet a test, is probably a good idea here.

[Geod24]

Is the @Name something provided by Configy? Or more importantly, does it expect the camelCase names in the JSON file, or the @Name ones? It's not really visible from the source code now, so at least a comment, or better yet a test, is probably a good idea here.

It expects the Name one. Name and Optional are in the attributes module.
Moved the code to a new module (since dub test ignores unittests in the app module) and added two tests covering hopefully all features.

[s-ludwig]

Okay, I see I got the previous behavior backwards, so this is good!