Added new helper function to ask user for a Yes/No answer
Now and error is raised if a config file does not exist

If the user explicitly specifies a path to a configuration file using
the -c or --config options an error will be raised if that file does not
exist at the given path.

If the default configuration file does not exist then this will be
silently ignored.
Added an epilog to the ArgumentParser instance that explains the use of default_config