Bot for saving contacts
Commands:
- hello - 'How can I help you?'
- add - add contact name and phone number
- delete - delete contact phone number
- change - change contact phone number
- add_birthday - add contact's birthday
- days_to_birthday' - show how many days are left until the contact's birthday
- phone - show contact phone number
- show all - show all contacts with phone numbers
- exit, close, good bye - close bot - 'Good bye!'