Refactors the code into separate components

[sortigoza]

This refactors the code to have 4 separate components:

• Config: parses and expose a consistent interface to read config values
• GitHubDataProvider: encapsulates all the GitHub fetching and filtering logic
• Formater: encapsulates all the text formatting logic
• Slack: used to send message to slack

This makes it easier to add different notification services beside slack and different formatting

Also, it adds logging and a dockerfile.