Extract getOpenMilestoneByTitleOrNull method in GitHubMilestoneManager and refactor getOpenMilestoneByTitle to use it. This method is also used in the new ChangelogGeneratorMain#createMilestone method.
Add createMilestone method to GitHubMilestoneManager.
Add -N / --create-next-milestone option in ChangelogGeneratorMain.
Add logic in ChangelogGeneratorMain to create the next milestone if the new option is provided at the command line.
Closes #51