If a posting was predicted by smart_importer or was entered by hand, it should be editable.
This was suggested and discussed in jbms#60

beancount-import will now look for new_account and unconfirmed_account
meta fields in each posting, and understand their presence to mean that
the account names of these postings are tentative, potentially new.

Introduced two new methods similar to those present in training.py
and new logic.