make gitlab-time-report available as CLI command

This makes the program's main routine available on the CLI:

• within the project as

  poetry run gitlab-time-report

• after successfull installation with pip install gitlab-time-report simply as

  gitlab-time-report

To try out the latter before this change is released on PyPI:

• download the wheel build from this branch

• pip install path/to/gitlab_time_report-1.0.1.dev0-py3-none-any.whl