Feature Proposal: Gitlab build "success_with_warning"

Hey guys,

with the advent of Gitlab 8.10 (particularly this MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5004) it is now possible to mark builds as "success with warning".

That's quite interesting for us, especially in connection to this plugin. Is anybody of you guys interested in something like this:

```
gitlabCommitStatus("static_analysis", warn_on_failure = true) {
// do some Sonar for example
}
```

Which would mean: if the sonar step fails (for example through the use of build breaker (or for whatever reason)), mark the build as warn, since it's not directly affecting production (at least not in a way that a failing test or even build would be).

If there are people interested in it, I may give it a shot.

Cheers,
Jan