fbmessenger: support config migration for fbmessengerexport source

for now kinda copied from reddit... still thinking about a more generic way

my/config.py

@@ -120,5 +120,7 @@ class hackernews:
 
 
 class fbmessenger:
+    class fbmessengerexport:
+        export_db: PathIsh
     class android:
         export_path: Paths

my/fbmessenger/export.py

@@ -7,15 +7,38 @@ REQUIRES = [
     'git+https://github.com/karlicoss/fbmessengerexport',
 ]
 
+from dataclasses import dataclass
 from pathlib import Path
 from typing import Iterator
 
-from ..core import PathIsh
+from my.config import fbmessenger as user_config
 
 import fbmessengerexport.dal as messenger
 
-from my.config import fbmessenger as config
-# TODO migrate this config to my.fbmessenger.export
+
+###
+# support old style config
+_new_section = getattr(user_config, 'fbmessengerexport', None)
+_old_attr    = getattr(user_config, 'export_db', None)
+
+if _new_section is None and _old_attr is not None:
+    from my.core.warnings import high
+    high("""DEPRECATED! Please modify your fbmessenger config to look like:
+
+class fbmessenger:
+    class fbmessengerexport:
+        export_db: PathIsh = '/path/to/fbmessengerexport/database'
+            """)
+    class fbmessengerexport:
+        export_db = _old_attr
+    setattr(user_config, 'fbmessengerexport', fbmessengerexport)
+###
+
+
+from ..core import PathIsh
+@dataclass
+class config(user_config.fbmessengerexport):
+    export_db: PathIsh
 
 
 def _dal() -> messenger.DAL: