Allow db to be provided separately from the app

flask_migrate/__init__.py

@@ -35,11 +35,13 @@ def get_template_directory(self):
 class Migrate(object):
     def __init__(self, app=None, db=None, directory='migrations', **kwargs):
         self.configure_callbacks = []
+        self.db = db
         self.directory = directory
         if app is not None and db is not None:
             self.init_app(app, db, directory, **kwargs)
 
-    def init_app(self, app, db, directory=None, **kwargs):
+    def init_app(self, app, db=None, directory=None, **kwargs):
+        self.db = db or self.db
         self.directory = directory or self.directory
         if not hasattr(app, 'extensions'):
             app.extensions = {}