Fix broken flask extension interface.

See http://flask.pocoo.org/docs/1.0/extensiondev/

The __init__ method takes an optional app object and, if supplied, will
call init_app.

Author: Boldyrev Yaroslav

flask_pymongo/__init__.py

@@ -108,7 +108,7 @@ class PyMongo(object):
 
     """
 
-    def __init__(self, app, uri=None, *args, **kwargs):
+    def __init__(self, app=None, uri=None, *args, **kwargs):
         self.cx = None
         self.db = None