Publisher for GitLab

### Preflight Checklist

* [x] I have read the [contribution documentation](https://github.com/electron-userland/electron-forge/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [code of conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project follows, as appropriate.
* [x] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem Description
I would like to have an auto-updater on my app, and I'm using GitLab for versioning. I haven't found any feature or instructions for this kind of set-up.

### Proposed Solution
Support for auto-updating using GitLab CI. And a demo on how to achieve it.