`boardgame-util stub` feature to add new delegate methods to main.go

[jkomoros]

For example, if you decide after you already have a game delegate that you want to add a ConfigureConstants(), ideally there's a stub command that allows you to do that

(run from examplegame folder) boardgame-util stub add constants

[jkomoros]

And if you run boardgame-util stub add (no arguments) then it gives you an interactive prompt to pick the one to add