Introduce Publisher and Resolver plugins

[iRevive]

My attempt to solve #44.

Perhaps githubRepositories should be renamed to githubResolveRepositories to make the self-description clearer?

The update should not be breaking for current users.

githubPublishToRepository and githubRepositories created from githubOwner, githubRepostiory, and githubTokenSource.

githubOwner, githubRepostiory, and githubTokenSource have deprecation warnings.

Publisher example

addSbtPlugin("com.codecommit" % "sbt-github-packages-publisher" % x)

import sbtghpackages.{GitHubRepository, TokenSource}
githubPublishToRepository := GitHubRepository("owner", "repo", TokenSource.Environment("GH_TOKEN"))

Even if a token is missing, only the publish step will fail.

Resolver example

addSbtPlugin("com.codecommit" % "sbt-github-packages-resolver" % x)

import sbtghpackages.{GitHubRepository, TokenSource}
githubRepositories += GitHubRepository("owner", "repo", TokenSource.Environment("GH_TOKEN"))

[iRevive]

If this concept is viable, I will improve docs further.