Add kwargs for configurable forms.

Specifically list out the kwargs so we'll get an interpreter error
on a bad name.

flask_security/core.py

@@ -184,7 +184,9 @@ def _get_state(app, datastore, **kwargs):
         _send_mail_task=None
     ))
 
-    kwargs.update(_default_forms)
+    for key, value in _default_forms.items():
+        if key not in kwargs or not kwargs[key]:
+            kwargs[key] = value
 
     return _SecurityState(**kwargs)
 
@@ -293,7 +295,11 @@ class Security(object):
         if app is not None and datastore is not None:
             self._state = self.init_app(app, datastore, **kwargs)
 
-    def init_app(self, app, datastore=None, register_blueprint=True, **kwargs):
+    def init_app(self, app, datastore=None, register_blueprint=True,
+        login_form=None, confirm_register_form=None,
+        register_form=None, forgot_password_form=None,
+        reset_password_form=None, send_confirmation_form=None,
+        passwordless_login_form=None):
         """Initializes the Flask-Security extension for the specified
         application and datastore implentation.
 
@@ -311,7 +317,14 @@ class Security(object):
 
         identity_loaded.connect_via(app)(_on_identity_loaded)
 
-        state = _get_state(app, datastore)
+        state = _get_state(app, datastore,
+                           login_form=login_form,
+                           confirm_register_form=confirm_register_form,
+                           register_form=register_form,
+                           forgot_password_form=forgot_password_form,
+                           reset_password_form=reset_password_form,
+                           send_confirmation_form=send_confirmation_form,
+                           passwordless_login_form=passwordless_login_form)
 
         if register_blueprint:
             app.register_blueprint(create_blueprint(state, __name__))