Refactor and test parts of core logic

[artem-y]

In this PR:

• moved the logic related to searching for issues and generating the commit message out into message_generator package
• added tests for the core logic of MessageGenerator
• refactored the config package, added interim commitConfigDTO struct for unmarshalling JSON:
  we don't need to have pointers in the CommitConfig after we detect all unset properties and replace them with default values, but the "" are valid values (except for the regex, but I'll work on that later)
• abstracted file-reading calls to the os package with FileReading interface, created a simple FileReader facade to just execute the same calls, and added a FileReadingMock to be used in unit tests
• added tests for the core logic of config creation, from file or default properties
• updated Makefile to run all tests from all the packages on make test command
• added unit testing workflow
• updated README, moved LICENSE to the root of the repo to make it appear in the "License" tab

What's Next:

There's much more refactoring left to do and more tests to write, I might get back to it in the future. Maybe I'll also add some step for code coverage.
I also want to add a simpler installation script and more safe-guards against some edge cases where the config file has invalid input.