Add --timestamp; makes timestamps optional but default off

[gominoa]

Suggest --no-timestamp pull request instead.

You pick.

[gominoa]

Or could make timestamp default on and change to '--no-timestamp' to hide

I prefer without timestamp as I log to file with --quiet and --verbose and my logger adds timestamp anyway. But rolling timestamp with hash rate looks sweet and most people probably dont log to file.

Will add another branch. You pick.