Rules to setting completion date when completing a task

Before 4.4.0 TTDL sets completion date for a completed task only if the task had creation date before completion. Now this behavior is defined by TTDL configuration file - see global.add_completion_date_always option, and by a command-line option --add-completion-date-always. By default the option is disabled, and TTDL works in the same way as it did in older versions.