#008 Implement new features and test them

[ronylpatil]

• F1 - time series (month vs no of msgs)
• F2 - heatmap (time vs weekday vs no of msgs)
• F3 - mostly used emojis
• F4 - idle user (0 messages)
• F5 - rarely active users
• manualy test in jupyter notebook
• format the code with black
• lint with mypy cmd: mypy src
• implement unit tests
• implement integration tests
• test with pytest cmd: pytest -v
• test with tox cmd: tox
• implement CI pipeline
• change development status in setup.py (from 3 - Alpha to 4 - Beta)
• update README.rst
• push the code to github