balance: --tree makes multicolumn reports show hierarchy

Multicolumn balance reports can now be switched to a hierarchical view
with --tree. This is similar to the single column balance report with
--no-elide, ie we do not elide boring parents into the following line.

The --flat and --tree flags are opposites; the last one to appear on
the command line is decisive (in future, if not already).
accounts: make accounts a built-in command, listing posted account names
balance: support --drop in flat multicolumn reports