Changes include:
- flat mode now shows exclusive (subaccount-excluding) balances.
This is a deviation from ledger, but seems simpler and clearer
for users and implementors across the various modes.
- in flat mode, --depth now aggregates deeper accounts at the
depth limit, rather than just excluding them from the report.
This is more useful.
- in flat mode, --empty no longer shows parent accounts with
no postings.
- more tests, more debug output, clearer code
