Support CatLight Protocol in Gitlab CI

Description

Support the CatLight Protocol GitLab CI.

Proposal

I as a developer want to receive build status notifications via CatLight.

Links / references

• http://blog.catlight.io/post/158337666873/open-protocol-for-integration-with-continuous
• https://github.com/catlightio/catlight-protocol

Documentation blurb

Overview

CatLight is a cross platform desktop application for receiving build status notifications. They've introduced the CatLight Protocol 1.0 seven months ago. There exists an feature proposal on the CatLight side, but it's nearly a year old and seems not to be noticed.

Use cases

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml