Overhaul

* Switch from gitlab to [PyGithub](https://github.com/jacquev6/PyGithub)
* Added command-line parsing and removed all hard-coded values
* Added support for getting credentials from git config
* Added support for rewriting ticket numbers into issue references
* Added support for migrating milestones
* Added tracking information to issues to avoid losing data which
  cannot be stored natively in Github
* Improved wiki syntax migration
* Added support for migrating ticket assignments