Add option to top interface to change update interval time

[pawamoy]

1. add a parameter update_interval to init method of class interface.Interface. This value is used to change the sleep value: self.sleep = update_interval / self.frames
   • Example with 1 second interval (default): sleep = 1 / 200 = 0.005
   • Example with 2 seconds interval: sleep = 2 / 200 = 0.01
2. add an option to the top subparser of the main CLI parser (in cli.get_parser function). Something like "-i", "--update-interval", type=float, default=1.0
3. write tests in tests/test_interface.py
4. update documentation