Allow disabling the choice of currency.

When 'none' is passed for the currency to the dialog method, the currency drop down shows '(Disabled)'.
Do not allow -X in debit/credit reports.

See also http://bugs.ledger-cli.org/show_bug.cgi?id=870.
Allow cash flow reports to show balances in a given currency.

Fix the formats so that the debit/credit balance report can be displayed in a specified currency.
Unfortunately, I have not been able to fix the register report (see http://bugs.ledger-cli.org/show_bug.cgi?id=870).