fix build failure by implementing cli.Flag

[mtgxx]

go get -u -v github.com/schollz/cowyodel fails with:

cowyodel/main.go:53:17: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
        cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)
cowyodel/main.go:60:15: cannot use cli.BoolFlag literal (type cli.BoolFlag) as type cli.Flag in array or slice literal:
        cli.BoolFlag does not implement cli.Flag (Apply method has pointer receiver)
cowyodel/main.go:66:15: cannot use []cli.Command literal (type []cli.Command) as type []*cli.Command in assignment
cowyodel/main.go:72:17: cannot use cli.BoolFlag literal (type cli.BoolFlag) as type cli.Flag in array or slice literal:
        cli.BoolFlag does not implement cli.Flag (Apply method has pointer receiver)
cowyodel/main.go:77:17: cannot use cli.BoolFlag literal (type cli.BoolFlag) as type cli.Flag in array or slice literal:
        cli.BoolFlag does not implement cli.Flag (Apply method has pointer receiver)
cowyodel/main.go:82:17: cannot use cli.BoolFlag literal (type cli.BoolFlag) as type cli.Flag in array or slice literal:
        cli.BoolFlag does not implement cli.Flag (Apply method has pointer receiver)
cowyodel/main.go:87:19: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
        cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)
cowyodel/main.go:187:19: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
        cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)```

This change fixes the issue.