Add GitLab Repository Support

Hi! I noticed that git2txt currently only supports GitHub repositories. I'd love to contribute by adding GitLab support to make the tool more versatile for the community.

According to the contribution guide, I wanted to check if this feature aligns with the project's vision before starting work on it. The implementation would:

- Add GitLab URL validation and parsing
- Support both gitlab.com and self-hosted instances
- Maintain the same output format and user experience
- Include appropriate tests
- Follow the existing code style

If this sounds good, I'll create a pull request following the contribution guidelines. Let me know if you'd like any adjustments to this approach.