Make an argument's value to a list if there are repeating value settings...

This is for #132, List-type arg values.

1. Prevent ParseMachine's property `waiting_for_flag_value` from stopping repeating values.
2. In Argument.set_value(), if the `value` is not empty, make it as a list and append the given value.

It's a really quick hack, I know, and look forward your feedback and idea because I really like to make this feature work ASAP :)