feature: metadata injection

[errordeveloper]

In some cases an application may need to be able to consume metadata generated by imagine, e.g. tag, so that it doesn't need to implement custom logic to lookup/inject version number.
It would be handly to define a well-known build argument that can be used at build time to be injected as a variable by a linker, or to set an environment variable at runtime. A JSON file injection might also be a plausible option.