ledger_entry: Only try to balance entry amounts when necessary.

This gives template authors the ability to write entries that
balance something other than the "main" amount.

ledger_entry: Support string tests in amount expressions.

This is useful for setting amounts based on imported strings like country,
Eventbrite ticket type, etc.

ledger_entry: Improve error reporting for amount expressions.

This is going to become more important as amount expressions become more
capable.

hooks.ledger_entry: Bring in all Ledger-specific code.

This includes the old "template" module, plus the associated
template-loading code from config.

hooks.ledger_entry: New hook to output Ledger entries.

This is roughly the smallest diff necessary to move output to a hook.
There's a lot of code reorganization that should still happen to bring it
better in line with this new structure.

template: Template amounts are now written with fuller expressions.

This makes it possible to support transactions that divide funds in
ways other than a simple percentage split. For example, there
might be tax withheld, or a flat fee expense on each transaction.

import2ledger: First version.

This is a pretty feature-complete version 1.
I don't know why I waited this long to commit anything.