Add Nix build and packaging support

[siriobalmelli]

I manage my system using Nix's home-manager, and I am adopting git-repo-manager to manage all of the Git repos I work with.

In this commit series I add support for:

• building and running git-repo-manager with Nix
• creating a build environment with all dependencies for git-repo-manager development
• importing git-repo-manager in other Nix builds (my personal use case)

I plan on continuing to use git-repo-manager personally, and in that case am also happy to maintain Nix support in this project.

[hakoerber]

Hey, this is awesome! Thank you for that.

I'm not familar with Nix. I'll take a closer look at this shortly and get back to you.

[hakoerber]

I'll merge this as-is. I won't be able to support Nix though, so I may have to ping you if any issues pop up :)

Thank you for this!

[siriobalmelli]

Thank you, I appreciate the responsiveness.

Happy to maintain Nix support, feel free to ping.