Proposal: stateful import

[xkikeg]

Summary

Current import is stateless, combines okane.yml and source and emits the Ledger format.
Stateful import instead relies on already eixsting Ledger file metadata to detect which files are already imported.

This requires #73 to be implemented beforehand, also requires some key designs:

1. How to encode file / position information
   Note each file may contain different semantics of the "unique record ID"
2. How to propagate the diff to the users, showing diff? log summary?