Idea: support augmentation of generated YAML

[janstenpickle]

While it's super handy generating workflows from SBT, Github are adding features to actions all the time and that presents a binary choice when wanting to use this plugin or a certain Github actions feature, if said feature hasn't been implemented here. An example of this is #7.

Kubernetes have the Kustomize tool, that allows for YAML fragments to be merged with the output of a template or base file. I was wondering if this might be a good idea for this plugin? I realise that this would be very much a "use at your own risk" feature, as people could end up breaking what would otherwise be a working workflow, but it would enable usage of the entire feature set from Github actions.