mirrorshades 0.2.0

Development release of mirrorshades.

* Move project to GitHub, the new project URL is
  <https://github.com/unnecessary-abstraction/mirrorshades>.

* Add GitHub mirroring support.

* Prune deleted branches when updating a mirrored git repository.

* Support multiple attempts when running custom commands.

* Add config validation using [desert](https://pypi.org/project/desert/)
  and [marshmallow](https://pypi.org/project/marshmallow/).

* Update and expand README file.

* Improve developer & contributor workflows
  with the addition of automatic linting & formatting.
  These checks are ran in GitHub Actions
  when contributing to the project via a pull request.
  They can also be ran locally using
  [pre-commit](https://pre-commit.com/).

* Satisfy the [REUSE Specification](https://reuse.software/spec/)
  to ensure licensing is clear.