Add Support for analyzing GitLab Repositories

[nickveenhof]

Would be great if we could change the API's to support more than just GitHub but also GitLab. Rearchitecture could potentially also support other Git platforms.

Disclosure: I'm from GitLab myself. I've asked someone internally to look into this given the source code.

[kaxada]

@nickveenhof we are looking forward to having such an integration. Would like to know who is looking into the issue so that we can collaborate closely.