support --with(out)-threads for compatibility

[kzidane]

In CS50 (Introduction to Computer Science) at Harvard we use Flask in the second half of the course. We have our own customized development environment (CS50 IDE) which is a cloud-based IDE based on Cloud9 that students are encouraged to use since it allows them to try any of the code demos if they want and start working on their assignments immediately without having to worry about setting up and configuring a local development environment.

In this environment we provide students with a wrapper around the flask CLI to abstract some of the complexities away from them. At the end of the course, students decide on a final project as their graduation project and they're allowed to use any environment(s) and toolset(s) that appeal. Some of them choose to work on the CS50 IDE and develop web apps that use Flask-SocketIO. But because our flask CLI wrapper passes --with-threads by default and Flask-SocketIO doesn't recognize this option, their flask runs fail.

I know Flask-SocketIO uses threads so --with-threads is implied, but just for compatibility with the original flask CLI, this PR just adds support for the --with(out)-threads dummy options.

cc @dmalan

#604

[miguelgrinberg]

Yes, this makes perfect sense. Thanks!

[dmalan]

Thank you, @miguelgrinberg!