v2.2.0

Features

• Add new option perform_once: true into observers.attach() to define an observer that will execute only once. #8
• Add new method observers.once() to define callables that will execute only once. #8
• Add new method observers.off() to remove observers' objects or to remove callables by their event names. #8

Improvement

• Allow defining multiple callables to the same event when their values are different. #9