Constrain examples names to valid names

[jasongitmail]

Example error output if user uses an invalid example name:

❯ zk example foo
zk example [name]

Create an example project

Options:
--name [string] [required] [choices: "sudoku", "tictactoe"]
-h, --help Show help [boolean]
-v, --version Show version number [boolean]

Invalid values:
name was "foo". Must be one of: "sudoku", "tictactoe".

(We don't have a lot of flexibility in the formatting of that error message because it must accept three string replacements. But I made it more readable than the default error message provided by yargs.)

[ymekuria]

LGTM. Thanks for adding this!