Refactor release notes generation to also work with git services other than github

[epeee]

Goal

Let's decouple release note generation from github and furthermore make it possible to use other git services.

Steps

• [ ] move github related code from org.shipkit.internal.notes.contributors to org.shipkit.internal.notes.contributors.github
• [ ] introcude interface for fetching contributors
• [ ] introduce GithubPlugin which will provide tasks for fetching contributor infos and release notes data from github
• [ ] ReleaseNotesPlugin should just take care about updating release notes and therefore should just create updateReleaseNotes task and configure it accordingly.

[mockitoguy]

This sound great! Thank you for looking into it! Feel free to submit PRs with some preliminary design / thoughts how to make it work.