Add gitlab support, with custom endpoint configuration

I would love to see gitlab support in this project, specially for self-hosted gitlab installations.

I don't care if it needs to be specified as a service configuration, in that case I would run my own badge-matrix service on our cloudstack.

But, if it can be configured directly via GET param can be awesome.

https://docs.gitlab.com/ee/user/project/pipelines/settings.html#badges

Start by reading the existing Travis and Sauce Labs integrations, then compare their configuration points with the linked GitLab badge documentation. Define how hosted GitLab and self-hosted endpoints should be selected, and consider both service configuration and GET parameters before verifying badges for each setup.

Written by the indexing model from the issue text.