added option to not output any text to terminal (if run in background)

I've added an option `-q` so that no text is printed to the terminal. This might be useful for people who want to run this script in the background.
IMO it should be the other way around: the text for the user should only be printed when requested (a separate -d debug option), but it's not my script, so I've just added a `-q` (quiet) option.