* Resolves#118
* Leverage flakes to manage package sources & hashes
* Update documentation with an example.
* Add `mkVersion` function to autogenerate a version string.
* Add srcs package via overlay containing all sources defined in
_pkgs/flake.nix_
* Extend `flk update` with the ability to only update the given input
* Update the readme to pull from cachix first, so users don't have to
build nix.
* Pull `nixFlakes` from override for slightly newer ui, and consistency
for users who change the nixos flake ref to something newer.
