Support other git sources

v0.3.0 2022-04-09

Major change(s):

Create a new shards.nix format that can be used by the nixpkgs fetchgit
fetcher instead of fetchFromGitHub to allow fetching from all git sources.

Minor change(s):

• Build using nix and provide flake.nix for an easy development experience.
• Add some basic tests

v0.2.0

The CLI functionality has been separated out from the core logic. This means we are now able to actually run tests against it.