Add GitLab support for updating merge requests

[Andrewpk]

After working on project with @stkent that utilized gnag, I'd really like to continue using it within our development org. After reviewing the gnag repo, it seems like supporting our internal GitLab server wouldn't be difficult.

This issue is a feature request, which if found acceptable, I would be happy to do the development for.
I had envisioned that this support would look very similar to the GitHub support currently included in gnag.

[btkelly]

Hmm, this would have to be a major structural change to the architecture of the plugin. I'm not opposed to changing Gnag to support multiple reporting systems but should probably be a larger discussion around GitHub abstraction before we can add GitLab.

[johnjohndoe]

I am also interested in support for GitLab as well as Stash/Bitbucket.

[tomasbjerre]

Regarding Gitlab:

• https://github.com/jenkinsci/violation-comments-to-gitlab-plugin
• https://github.com/tomasbjerre/violation-comments-to-gitlab-maven-plugin
• https://github.com/jenkinsci/violation-comments-to-gitlab-plugin

And Bitbucket / Stash:

• https://wiki.jenkins.io/display/JENKINS/Violation+Comments+to+Bitbucket+Server+Plugin