Group does not work for endpoints without classes

[fizaashraf37]

If endpoints are defined inside a class as in below example. Then proper tag is assigned to endpoint for grouping.

class TestView(BaseApi):
        @app.route('/objects/<int:object_id>', methods=['PUT'])
        def update_object(object_id):
                return jsonify({'id': 1, 'name': 'test_object_name'}), 201

In this case Tag "TestView" is assigned to '/objects/int:object_id' route.

But if the endpoint is defined without any class, then no Tag is assigned to the endpoint. It comes under None category. In this case user should have the option to define tag for the route.

[fizaashraf37]

I have created a feature branch for this and also created PR#27