Add option for --effective-date

This patch allows for the specification of CSV column that contains a secondary date column such as a 'Posted Date'.  Use of this option requires a custom template file.

I have hopefully updated all the right bits of the script as well as the help file.

The formatting of the effective date assumes both that the CSV only contains one date format AND that both the transaction date and effective date should be in the same ledger format.  This conversion could probably be coded more cleanly, I'm open to suggestions.