Previously, we had two places where we were passing kwargs that affected
the image generation: the ImageSpec constructor and the generate method.
These were essentially partial applications. With this commit, there's
only one partial application (when the spec is instantiated), and the
generate method is called without arguments. Therefore, specs can now
be treated as generic generators whose constructors just happen to
accept a source_file.
The registry module splits the work that specs.SpecRegistry
used to do into two classes: GeneratorRegistry and
SourceGroupRegistry. These two registries are wrapped in
Register and Unregister utilities for API convenience.
