Documentation improvements.
Add option --ignore-transactions-outside-dates.

This is mainly useful so that we can optionally ignore transactions
outside specified dates when showing the running balances on an account.

If this is specified, we ignore any transactions before the <first-date>
and the account's balance starts at zero. If this is not specified,
transactions occuring before <first-date> will affect the account's,
balance but we won't bother printing them, and the balance may not start
from zero.
Documentation improvements.