socketio cli run command stomps on flask's own cli run command

[arabidopsis]

With flask_socketio installed, running a non-socketIO application with 'flask run' still invokes
flask_socketio.cli:run command. Which then fails when fetching app.extensions['socketio']

This is possibly a 'feature' not a bug. Maybe just a check
that app.extensions['socketio'] exists and otherwise run "normally"

[arabidopsis]

Awesome work! Thanks for that.