Make date not required for add command

[Sangeeta Jadoonanan]

For the groc add command, you can either enter details for one purchase using the --date,
--total, --store, --description flags or enter a path using the flag --source to point to a csv file or directory of csv files to enter.

For the one off purchase entry, the description is optional. The date should be optional too and if not passed, default to the current date.

[Sangeeta Jadoonanan]

Closed #2.