add option to list command to show sum per tag

[drademann]

The list command already has the -t (--tags) option to show the total hours per tag based on a day.

A new option should show the total hours per tag based on a month.

$ haora list -t month

The -t option could change from a boolean flag to a string flag.

• Given no value, the default period of a day is used. Given a
• Given month should use a month as period. The date the -d flag sets defines the month, or the current month when absent.
• Optional: Given week should use a week as period.

(month and week may be abbreviated)