Implement add/local-publish variants

[freekh]

Tentative API call:

repository.pull()
val gitFiles = adept.localPublish(variants, repository, artifacts = Set(new File("/Users/awesomeperson/Projects/myproject")))
repository.add(gitFiles)
repository.commit("A new release of my awesome project!")

[freekh]

Changed to:

def localPublish(variant: Variant, context: Set[ContextValue], repository: GitRepository, artifacts: Set[File]): Set[File] = {

because we must write context as well, and it is per variant.

Single variant publish is also more composable.