public @NonNull <T> Builder<C> flag(final CommandFlag.@NonNull Builder<C, T> builder) {
            return this.flag(builder.build());
        }
Registers a new command flag.

@param builder flag builder. {@link CommandFlag.Builder#build()} will be invoked.
@param <T>     flag value type
@return new builder instance that uses the provided flag
cloud-core/src/main/java/org/incendo/cloud/Command.java