Option to select grouping date

[woj-tek]

This is a follow-up to #421 (and kinda related to #360 I guess) - basically we can now enter charge date (which is great) but… we can't group by this date, so if we want to quickly compare bank statement for the month (INs and OUTs) we are still out of luck because bank will use actual charge time and not transaction time, and MyExpenses uses the transaction time for the grouping.