improve `gsh stat` command

[phyver]

A command to display statistics about missions.

• gsh stat session: number of completed / skipped / cancelled missions, duration of game, etc.
• gsh stat missions: number of tries for each mission, average time for succesful check, etc.
• gsh stat mission [N]: detailled statistic for given mission

That can easily be done using awk on the $GSH_ROOT/.config/missions.log file.

[phyver]

4473194
and
90dc0e2

That can probably be improved though...