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.

hooks: run() return value controls processing of entry data.

Instead of using in-band signaling with the entry_data dict.
I don't know why I didn't think of this in the first place.

hooks: Filter entries by setting entry_data['_hook_cancel'] to True.

The current method only works for plain dicts and other simple mappings.
Mapping that may still contain items after .clear(), like ChainMap, can't
rely on the old method.

hooks.add_entity: Better handle common name prefix parts.

Keep these with the name they're attached to, rather than breaking the name
in the middle.

import2ledger: First version.

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