Reorganize Project-Structure

[TheSnoozer]

• Move the core-plugin stuff to it's own github project
• Remove the core-plugin stuff from this project
• Flatten the project structure so it's essentially just the git-commit-id-maven-plugin

[TheSnoozer]

Would need to move the remaining core-tests from https://github.com/git-commit-id/git-commit-id-maven-plugin/tree/master/src/test/java/pl/project13/core to https://github.com/git-commit-id/git-commit-id-plugin-core