Add flag validators #2

    private static final Flag<Integer> angle = Flags.integer('angle')
        .validate(new Validator<Integer>() {
            @Override public boolean check(String flagName, Integer value) {
                return value >=0 && value < 360;
            }
        });

    private static final Flag<Integer> angle = Flags.integer('angle')
        .validate((flagName, value) -> value >=0 && value < 360);