[CLI] Add a --quiet option

[Kernald]

Even when running without errors, the CLI client is verbose:

'afterwriting command line interface
www: http://afterwriting.com

Loading script: screenplay.fountain
Generating PDF screenplay.pdf
Done!

It would be great to control these outputs with a --quiet/--verbose pair of flags. It's good practice for non-interactive CLI tools to not output anything when everything works fine and the process is fairly short - afterwriting arguably meets those criteria.