runserver: Improve handling of '-d' and '-r' switches

* debugger defaults to OFF because that's the safe choice
* reloader defaults to whatever the debugger default is
  if not set explicitly
Default use_debugger and use_reloader to app.debug
runserver: Debugging now defaults to whatever the app's configuration says

or True if that's None. Also, print a warning when debugging is on.
python3-ism needs __future__ import
/* Merge branch 'master' of ssh://github.com/smurfix/flask-script */
Set submanager's app instance.