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

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

config: Make get_section a public method.

As hooks and importers both require more configuration, they're gonna need
to have lower-level access to configuration settings.

config: Add open_output_file method.

Make this functionality accessible to hooks.

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.