Add UNCONFORMED_ACCOUNT_KEY to importer entries

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

beancount_import/source/generic_importer_source.py

@@ -29,6 +29,7 @@
 from .description_based_source import DescriptionBasedSource, get_pending_and_invalid_entries
 from .mint import _get_key_from_posting
 
+UNCONFIRMED = "unconfirmed_account"
 
 class ImporterSource(DescriptionBasedSource):
     def __init__(self,
@@ -64,7 +65,7 @@ def prepare(self, journal: 'JournalEditor', results: SourceResults) -> None:
             hashed_entries = OrderedDict() #type: Dict[Hashable, Directive]
             for entry in f_entries:
                 key_ = self._get_key_from_imported_entry(entry)
-                self._add_description(entry)
+                self._add_description_and_unconfirmed_account_key(entry)
                 hashed_entries.setdefault(key_, []).append(entry)
             # deduplicate across statements
             for key_ in hashed_entries:
@@ -84,24 +85,20 @@ def prepare(self, journal: 'JournalEditor', results: SourceResults) -> None:
             make_import_result=self._make_import_result,
             results=results)
 
-    def _add_description(self, entry: Transaction):
+    def _add_description_and_unconfirmed_account_key(self, entry: Transaction):
         if not isinstance(entry, Transaction): return None
-        postings = entry.postings #type: List[Posting]
-        to_mutate = []
-        for i, posting in enumerate(postings):
-            if posting.account != self.account: continue
-            if isinstance(posting.meta, dict):
+        for i, posting in enumerate(entry.postings):
+            if not isinstance(posting.meta, dict):
+                # replace posting with a clone with meta dict
+                entry.postings[i] = posting._replace(meta={})
+                posting = entry.postings[i]
+            if posting.account == self.account:
+                # add description
                 posting.meta["source_desc"] = entry.narration
                 posting.meta["date"] = entry.date
-                break
             else:
-                to_mutate.append(i)
-                break
-        for i in to_mutate:
-            p = postings.pop(i)
-            p = Posting(p.account, p.units, p.cost, p.price, p.flag,
-                        {"source_desc":entry.narration, "date": entry.date})
-            postings.insert(i, p)
+                # add unconfirmed account key
+                posting[UNCONFIRMED] = ""
 
     def _get_source_posting(self, entry:Transaction) -> Optional[Posting]:
         for posting in entry.postings: