Add activity calendar by author

[tomice]

The original git-quick-stats project now has the "Activity Calendar by Author" feature where it shows a shaded heatmap of how many commits have been made per month by a given author. It uses the short arg -k and long arg --commits-calendar-by-author. This brings about a new menu type called "Calendar" after the "Suggest" command. Pictures have been included to showcase example.

Port this feature to git-py-stats so that it maintains similar functionality.