* add 'version' command - "go-xkcdreader version" returns a formatted
version of the app
* test (among other things) that the version in flake.nix matches the
version hardcoded in app's go code (cmd/version.go)
* reformat the flake
* create a fyne application, specify deps as both buildGoModule
buildInputs and packages in devShells
* add nixGL overlay to devShells so that openGL-based apps (such as fyne
apps) run fine for non-NixOS users
* add build/link flags