Support save flag for countdown

[samit22]

• Add a save flag for the countdown command to save the countdown dates.
  Saving saves the data in file ~/.calendarN/countdowns/countdowns.txt

Data format Name of countdown date of the countdown and time of the countdown(optional)

"My Birthday" "2022-08-18" "00:00:00"

• countdown all -> Shows all saved countdowns in future
• countdown delete --name=<name_of_ct> deletes the countdown with given name
• countdown show --name=<name_of_ct> --run should support
  shows the specific countdown