Fix for #374 optimized -p option by throttling the output

[anindyasen]

#374 With -p option it will only print the percentage when there is a change in value.

[anindyasen]

Hi @ismaell Can you please review this PR?
Thank you.

[ismaell]

There's two issues with this patch:

1. It changes the behaviour of the output, it need s to be newline-terminated and with no prefix like before.
2. It expects a change in the progress number, which is undesirable, because output constitutes a keep-alive signal as well.

I favour a time-driven throttling mechanism instead.