Gitlab: work items

Gitlab [recently introduced Work Items](https://about.gitlab.com/handbook/product/cross-stage-features/work-items/), used to subdivide issues into smaller tasks.

I've started using them on Gitlab. While they get assigned identifiers in the same space as issues (so, work items numbers are just issue numbers), they don't show up in Bugwarrior. It would be nice if they could show up. Additionally, bugwarrior could also register the dependencies between a work-item and its parent issue!