Implement option for manual update of progressbar

This was another thing I found practical - The ability to control when the write occurred. As an "advanced" option, it defaults to false.

My reason for needing this, is that I sometimes need to provide a prompt to the user during the progress, which could collide with an update. I also happen to have a flow of updates that don't quite need the rate-limiting the builtin solution provides, but also benefits from instant updating.

Anyhow, I hope it's acceptable, especially seeing that the default behaviour remains the same.